diff --git a/fluent/migrate/__init__.py b/fluent/migrate/__init__.py
index 2a92dd5e..3582468d 100644
--- a/fluent/migrate/__init__.py
+++ b/fluent/migrate/__init__.py
@@ -1,6 +1,9 @@
 # coding=utf8
 
 from .context import MergeContext                      # noqa: F401
+from .errors import (                                  # noqa: F401
+    MigrationError, NotSupportedError, UnreadableReferenceError
+)
 from .transforms import (                              # noqa: F401
     Source, COPY, REPLACE_IN_TEXT, REPLACE, PLURALS, CONCAT
 )
diff --git a/fluent/migrate/context.py b/fluent/migrate/context.py
index 4c8a8c44..974f2cbe 100644
--- a/fluent/migrate/context.py
+++ b/fluent/migrate/context.py
@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 
 import os
+import sys
 import codecs
 import logging
 
@@ -18,7 +19,7 @@ def getParser(path):
 from .cldr import get_plural_categories
 from .transforms import Source
 from .merge import merge_resource
-from .util import get_message
+from .errors import NotSupportedError, UnreadableReferenceError
 
 
 class MergeContext(object):
@@ -79,6 +80,10 @@ def read_ftl_resource(self, path):
         f = codecs.open(path, 'r', 'utf8')
         try:
             contents = f.read()
+        except UnicodeDecodeError as err:
+            logger = logging.getLogger('migrate')
+            logger.warn('Unable to read file {}: {}'.format(path, err))
+            raise err
         finally:
             f.close()
 
@@ -94,7 +99,7 @@ def read_ftl_resource(self, path):
             logger = logging.getLogger('migrate')
             for annot in annots:
                 msg = annot.message
-                logger.warn(u'Syntax error in {}: {}'.format(path, msg))
+                logger.warn('Syntax error in {}: {}'.format(path, msg))
 
         return ast
 
@@ -105,52 +110,33 @@ def read_legacy_resource(self, path):
         # Transform the parsed result which is an iterator into a dict.
         return {entity.key: entity.val for entity in parser}
 
-    def add_reference(self, path, realpath=None):
-        """Add an FTL AST to this context's reference resources."""
-        fullpath = os.path.join(self.reference_dir, realpath or path)
-        try:
-            ast = self.read_ftl_resource(fullpath)
-        except IOError as err:
-            logger = logging.getLogger('migrate')
-            logger.error(u'Missing reference file: {}'.format(path))
-            raise err
-        except UnicodeDecodeError as err:
-            logger = logging.getLogger('migrate')
-            logger.error(u'Error reading file {}: {}'.format(path, err))
-            raise err
-        else:
-            self.reference_resources[path] = ast
+    def maybe_add_localization(self, path):
+        """Add a localization resource to migrate translations from.
 
-    def add_localization(self, path):
-        """Add an existing localization resource.
+        Only legacy resources can be added as migration sources.  The resource
+        may be missing on disk.
 
-        If it's an FTL resource, add an FTL AST.  Otherwise, it's a legacy
-        resource.  Use a compare-locales parser to create a dict of (key,
-        string value) tuples.
+        Uses a compare-locales parser to create a dict of (key, string value)
+        tuples.
         """
-        fullpath = os.path.join(self.localization_dir, path)
-        if fullpath.endswith('.ftl'):
-            try:
-                ast = self.read_ftl_resource(fullpath)
-            except IOError:
-                logger = logging.getLogger('migrate')
-                logger.warn(u'Missing localization file: {}'.format(path))
-            except UnicodeDecodeError as err:
-                logger = logging.getLogger('migrate')
-                logger.warn(u'Error reading file {}: {}'.format(path, err))
-            else:
-                self.localization_resources[path] = ast
+        if path.endswith('.ftl'):
+            error_message = (
+                'Migrating translations from Fluent files is not supported '
+                '({})'.format(path))
+            logging.getLogger('migrate').error(error_message)
+            raise NotSupportedError(error_message)
+
+        try:
+            fullpath = os.path.join(self.localization_dir, path)
+            collection = self.read_legacy_resource(fullpath)
+        except IOError:
+            logger = logging.getLogger('migrate')
+            logger.warn('Missing localization file: {}'.format(path))
         else:
-            try:
-                collection = self.read_legacy_resource(fullpath)
-            except IOError:
-                logger = logging.getLogger('migrate')
-                logger.warn(u'Missing localization file: {}'.format(path))
-            else:
-                self.localization_resources[path] = collection
+            self.localization_resources[path] = collection
 
-    def add_transforms(self, path, transforms):
-        """Define transforms for path.
+    def add_transforms(self, path, reference, transforms):
+        """Define transforms for path using reference as template.
 
         Each transform is an extended FTL node with `Transform` nodes as some
         values.  Transforms are stored in their lazy AST form until
@@ -165,6 +151,22 @@ def get_sources(acc, cur):
                 acc.add((cur.path, cur.key))
             return acc
 
+        refpath = os.path.join(self.reference_dir, reference)
+        try:
+            ast = self.read_ftl_resource(refpath)
+        except IOError as err:
+            error_message = 'Missing reference file: {}'.format(refpath)
+            logging.getLogger('migrate').error(error_message)
+            raise UnreadableReferenceError(error_message)
+        except UnicodeDecodeError as err:
+            error_message = 'Error reading file {}: {}'.format(refpath, err)
+            logging.getLogger('migrate').error(error_message)
+            raise UnreadableReferenceError(error_message)
+        else:
+            # The reference file will be used by the merge function as
+            # a template for serializing the merge results.
+            self.reference_resources[path] = ast
+
         for node in transforms:
             # Scan `node` for `Source` nodes and collect the information they
             # store into a set of dependencies.
@@ -175,17 +177,30 @@ def get_sources(acc, cur):
         path_transforms = self.transforms.setdefault(path, [])
         path_transforms += transforms
 
+        if path not in self.localization_resources:
+            fullpath = os.path.join(self.localization_dir, path)
+            try:
+                ast = self.read_ftl_resource(fullpath)
+            except IOError:
+                logger = logging.getLogger('migrate')
+                logger.info(
+                    'Localization file {} does not exist and '
+                    'it will be created'.format(path))
+            except UnicodeDecodeError:
+                logger = logging.getLogger('migrate')
+                logger.warn(
+                    'Localization file {} will be re-created and some '
+                    'translations might be lost'.format(path))
+            else:
+                self.localization_resources[path] = ast
+
     def get_source(self, path, key):
-        """Get an entity value from the localized source.
+        """Get an entity value from a localized legacy source.
 
         Used by the `Source` transform.
         """
-        if path.endswith('.ftl'):
-            resource = self.localization_resources[path]
-            return get_message(resource.body, key)
-        else:
-            resource = self.localization_resources[path]
-            return resource.get(key, None)
+        resource = self.localization_resources[path]
+        return resource.get(key, None)
 
     def merge_changeset(self, changeset=None):
         """Return a generator of FTL ASTs for the changeset.
@@ -200,10 +215,11 @@ def merge_changeset(self, changeset=None):
         """
 
         if changeset is None:
-            # Merge all known legacy translations.
+            # Merge all known legacy translations. Used in tests.
             changeset = {
                 (path, key)
                 for path, strings in self.localization_resources.iteritems()
+                if not path.endswith('.ftl')
                 for key in strings.iterkeys()
             }
 
@@ -240,10 +256,15 @@ def in_changeset(ident):
                 self, reference, current, transforms, in_changeset
             )
 
-            # If none of the transforms is in the given changeset, the merged
-            # snapshot is identical to the current translation. We compare
-            # JSON trees rather then use filtering by `in_changeset` to account
-            # for translations removed from `reference`.
+            # Skip this path if the merged snapshot is identical to the current
+            # state of the localization file. This may happen when:
+            #
+            #   - none of the transforms is in the changset, or
+            #   - all messages which would be migrated by the context's
+            #     transforms already exist in the current state.
+            #
+            # We compare JSON trees rather then use filtering by `in_changeset`
+            # to account for translations removed from `reference`.
             if snapshot.to_json() == current.to_json():
                 continue
 
diff --git a/fluent/migrate/errors.py b/fluent/migrate/errors.py
new file mode 100644
index 00000000..e52984f0
--- /dev/null
+++ b/fluent/migrate/errors.py
@@ -0,0 +1,10 @@
+class MigrationError(ValueError):
+    pass
+
+
+class NotSupportedError(MigrationError):
+    pass
+
+
+class UnreadableReferenceError(MigrationError):
+    pass
diff --git a/fluent/migrate/merge.py b/fluent/migrate/merge.py
index 3112cb54..586393e5 100644
--- a/fluent/migrate/merge.py
+++ b/fluent/migrate/merge.py
@@ -11,9 +11,10 @@ def merge_resource(ctx, reference, current, transforms, in_changeset):
     """Transform legacy translations into FTL.
 
     Use the `reference` FTL AST as a template.  For each en-US string in the
-    reference, first check if it's in the currently processed changeset with
-    `in_changeset`; then check for an existing translation in the current FTL
-    `localization` or for a migration specification in `transforms`.
+    reference, first check for an existing translation in the current FTL
+    `localization` and use it if it's present; then if the string has
+    a transform defined in the migration specification and if it's in the
+    currently processed changeset, evaluate the transform.
     """
 
     def merge_body(body):
diff --git a/fluent/migrate/transforms.py b/fluent/migrate/transforms.py
index 942fbd90..b747a128 100644
--- a/fluent/migrate/transforms.py
+++ b/fluent/migrate/transforms.py
@@ -66,6 +66,7 @@
 from __future__ import unicode_literals
 
 import fluent.syntax.ast as FTL
+from .errors import NotSupportedError
 
 
 def pattern_from_text(value):
@@ -113,6 +114,11 @@ class Source(Transform):
     # not be replaced?
 
     def __init__(self, path, key):
+        if path.endswith('.ftl'):
+            raise NotSupportedError(
+                'Migrating translations from Fluent files is not supported '
+                '({})'.format(path))
+
         self.path = path
         self.key = key
 
diff --git a/tests/migrate/fixtures/en-US/privacy.ftl b/tests/migrate/fixtures/en-US/privacy.ftl
new file mode 100644
index 00000000..bedd5a7c
--- /dev/null
+++ b/tests/migrate/fixtures/en-US/privacy.ftl
@@ -0,0 +1,9 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+dnt-description =
+    Send websites a “Do Not Track” signal
+    that you don’t want to be tracked
+dnt-learn-more = Learn more
+dnt-always = Always
diff --git a/tests/migrate/fixtures/pl/privacy.dtd b/tests/migrate/fixtures/pl/privacy.dtd
new file mode 100644
index 00000000..36878d2f
--- /dev/null
+++ b/tests/migrate/fixtures/pl/privacy.dtd
@@ -0,0 +1,7 @@
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this
+   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
+
+<!ENTITY doNotTrack.description "Old Description in Polish">
+<!ENTITY doNotTrack.learnMore.label "Więcej informacji">
+<!ENTITY doNotTrack.always.label "Zawsze">
diff --git a/tests/migrate/fixtures/pl/privacy.ftl b/tests/migrate/fixtures/pl/privacy.ftl
new file mode 100644
index 00000000..772da74e
--- /dev/null
+++ b/tests/migrate/fixtures/pl/privacy.ftl
@@ -0,0 +1,5 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+dnt-description = New Description in Polish
diff --git a/tests/migrate/test_concat.py b/tests/migrate/test_concat.py
index e63f41c2..48a4626e 100644
--- a/tests/migrate/test_concat.py
+++ b/tests/migrate/test_concat.py
@@ -16,6 +16,8 @@
 
 class MockContext(unittest.TestCase):
     def get_source(self, path, key):
+        # Ignore path (test.properties) and get translations from self.strings
+        # defined in setUp.
         return self.strings.get(key, None).val
 
 
@@ -36,7 +38,7 @@ def test_concat_one(self):
         msg = FTL.Message(
             FTL.Identifier('hello'),
             value=CONCAT(
-                COPY(self.strings, 'hello'),
+                COPY('test.properties', 'hello'),
             )
         )
 
@@ -51,8 +53,8 @@ def test_concat_two(self):
         msg = FTL.Message(
             FTL.Identifier('hello'),
             value=CONCAT(
-                COPY(self.strings, 'hello.start'),
-                COPY(self.strings, 'hello.end'),
+                COPY('test.properties', 'hello.start'),
+                COPY('test.properties', 'hello.end'),
             )
         )
 
@@ -86,8 +88,8 @@ def test_concat_whitespace_begin(self):
         msg = FTL.Message(
             FTL.Identifier('hello'),
             value=CONCAT(
-                COPY(self.strings, 'whitespace.begin.start'),
-                COPY(self.strings, 'whitespace.begin.end'),
+                COPY('test.properties', 'whitespace.begin.start'),
+                COPY('test.properties', 'whitespace.begin.end'),
             )
         )
 
@@ -103,8 +105,8 @@ def test_concat_whitespace_end(self):
         msg = FTL.Message(
             FTL.Identifier('hello'),
             value=CONCAT(
-                COPY(self.strings, 'whitespace.end.start'),
-                COPY(self.strings, 'whitespace.end.end'),
+                COPY('test.properties', 'whitespace.end.start'),
+                COPY('test.properties', 'whitespace.end.end'),
             )
         )
 
@@ -129,11 +131,11 @@ def test_concat_literal(self):
         msg = FTL.Message(
             FTL.Identifier('update-failed'),
             value=CONCAT(
-                COPY(self.strings, 'update.failed.start'),
+                COPY('test.properties', 'update.failed.start'),
                 FTL.TextElement('<a>'),
-                COPY(self.strings, 'update.failed.linkText'),
+                COPY('test.properties', 'update.failed.linkText'),
                 FTL.TextElement('</a>'),
-                COPY(self.strings, 'update.failed.end'),
+                COPY('test.properties', 'update.failed.end'),
             )
         )
 
@@ -157,9 +159,9 @@ def test_concat_replace(self):
         msg = FTL.Message(
             FTL.Identifier('channel-desc'),
             value=CONCAT(
-                COPY(self.strings, 'channel.description.start'),
+                COPY('test.properties', 'channel.description.start'),
                 FTL.Placeable(EXTERNAL_ARGUMENT('channelname')),
-                COPY(self.strings, 'channel.description.end'),
+                COPY('test.properties', 'channel.description.end'),
             )
         )
 
@@ -187,7 +189,7 @@ def test_concat_replace(self):
             FTL.Identifier('community'),
             value=CONCAT(
                 REPLACE(
-                    self.strings,
+                    'test.properties',
                     'community.start',
                     {
                         '&brandShortName;': MESSAGE_REFERENCE(
@@ -197,7 +199,7 @@ def test_concat_replace(self):
                 ),
                 FTL.TextElement('<a>'),
                 REPLACE(
-                    self.strings,
+                    'test.properties',
                     'community.mozillaLink',
                     {
                         '&vendorShortName;': MESSAGE_REFERENCE(
@@ -206,11 +208,11 @@ def test_concat_replace(self):
                     }
                 ),
                 FTL.TextElement('</a>'),
-                COPY(self.strings, 'community.middle'),
+                COPY('test.properties', 'community.middle'),
                 FTL.TextElement('<a>'),
-                COPY(self.strings, 'community.creditsLink'),
+                COPY('test.properties', 'community.creditsLink'),
                 FTL.TextElement('</a>'),
-                COPY(self.strings, 'community.end')
+                COPY('test.properties', 'community.end')
             )
         )
 
diff --git a/tests/migrate/test_context.py b/tests/migrate/test_context.py
index 775db8f2..4b7eef32 100644
--- a/tests/migrate/test_context.py
+++ b/tests/migrate/test_context.py
@@ -7,6 +7,7 @@
 
 import fluent.syntax.ast as FTL
 
+from fluent.migrate.errors import NotSupportedError, UnreadableReferenceError
 from fluent.migrate.util import ftl, ftl_resource_to_json, to_json
 from fluent.migrate.context import MergeContext
 from fluent.migrate.transforms import COPY
@@ -25,15 +26,14 @@ def setUp(self):
             localization_dir=here('fixtures/pl')
         )
 
-        self.ctx.add_reference('aboutDownloads.ftl')
         try:
-            self.ctx.add_localization('aboutDownloads.dtd')
-            self.ctx.add_localization('aboutDownloads.properties')
+            self.ctx.maybe_add_localization('aboutDownloads.dtd')
+            self.ctx.maybe_add_localization('aboutDownloads.properties')
         except RuntimeError:
             self.skipTest('compare-locales required')
 
     def test_hardcoded_node(self):
-        self.ctx.add_transforms('aboutDownloads.ftl', [
+        self.ctx.add_transforms('aboutDownloads.ftl', 'aboutDownloads.ftl', [
             FTL.Message(
                 id=FTL.Identifier('about'),
                 value=FTL.Pattern([
@@ -58,7 +58,7 @@ def test_hardcoded_node(self):
         )
 
     def test_merge_single_message(self):
-        self.ctx.add_transforms('aboutDownloads.ftl', [
+        self.ctx.add_transforms('aboutDownloads.ftl', 'aboutDownloads.ftl', [
             FTL.Message(
                 id=FTL.Identifier('title'),
                 value=COPY(
@@ -84,7 +84,7 @@ def test_merge_single_message(self):
         )
 
     def test_merge_one_changeset(self):
-        self.ctx.add_transforms('aboutDownloads.ftl', [
+        self.ctx.add_transforms('aboutDownloads.ftl', 'aboutDownloads.ftl', [
             FTL.Message(
                 id=FTL.Identifier('title'),
                 value=COPY(
@@ -123,7 +123,7 @@ def test_merge_one_changeset(self):
         )
 
     def test_merge_two_changesets(self):
-        self.ctx.add_transforms('aboutDownloads.ftl', [
+        self.ctx.add_transforms('aboutDownloads.ftl', 'aboutDownloads.ftl', [
             FTL.Message(
                 id=FTL.Identifier('title'),
                 value=COPY(
@@ -176,7 +176,7 @@ def test_merge_two_changesets(self):
         self.assertDictEqual(merged_b, expected_b)
 
     def test_serialize_changeset(self):
-        self.ctx.add_transforms('aboutDownloads.ftl', [
+        self.ctx.add_transforms('aboutDownloads.ftl', 'aboutDownloads.ftl', [
             FTL.Message(
                 id=FTL.Identifier('title'),
                 value=COPY(
@@ -245,8 +245,8 @@ def tearDown(self):
         logging.disable(logging.NOTSET)
 
     def test_missing_reference_file(self):
-        with self.assertRaises(IOError):
-            self.ctx.add_reference('missing.ftl')
+        with self.assertRaises(UnreadableReferenceError):
+            self.ctx.add_transforms('some.ftl', 'missing.ftl', [])
 
 
 class TestIncompleteLocalization(unittest.TestCase):
@@ -260,13 +260,12 @@ def setUp(self):
             localization_dir=here('fixtures/pl')
         )
 
-        self.ctx.add_reference('toolbar.ftl')
         try:
-            self.ctx.add_localization('browser.dtd')
+            self.ctx.maybe_add_localization('browser.dtd')
         except RuntimeError:
             self.skipTest('compare-locales required')
 
-        self.ctx.add_transforms('toolbar.ftl', [
+        self.ctx.add_transforms('toolbar.ftl', 'toolbar.ftl', [
             FTL.Message(
                 id=FTL.Identifier('urlbar-textbox'),
                 attributes=[
@@ -309,3 +308,122 @@ def test_missing_localization_file(self):
             to_json(self.ctx.merge_changeset()),
             expected
         )
+
+
+class TestExistingTarget(unittest.TestCase):
+    def setUp(self):
+        # Silence all logging.
+        logging.disable(logging.CRITICAL)
+
+        self.ctx = MergeContext(
+            lang='pl',
+            reference_dir=here('fixtures/en-US'),
+            localization_dir=here('fixtures/pl')
+        )
+
+        try:
+            self.ctx.maybe_add_localization('privacy.dtd')
+        except RuntimeError:
+            self.skipTest('compare-locales required')
+
+    def tearDown(self):
+        # Resume logging.
+        logging.disable(logging.NOTSET)
+
+    def test_existing_target_ftl_missing_string(self):
+        self.ctx.add_transforms('privacy.ftl', 'privacy.ftl', [
+            FTL.Message(
+                id=FTL.Identifier('dnt-learn-more'),
+                value=COPY(
+                    'privacy.dtd',
+                    'doNotTrack.learnMore.label'
+                )
+            ),
+        ])
+
+        expected = {
+            'privacy.ftl': ftl_resource_to_json('''
+        // This Source Code Form is subject to the terms of the Mozilla Public
+        // License, v. 2.0. If a copy of the MPL was not distributed with this
+        // file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+        dnt-description = New Description in Polish
+        dnt-learn-more = Więcej informacji
+            ''')
+        }
+
+        self.maxDiff = None
+        self.assertDictEqual(
+            to_json(self.ctx.merge_changeset()),
+            expected
+        )
+
+    def test_existing_target_ftl_existing_string(self):
+        self.ctx.add_transforms('privacy.ftl', 'privacy.ftl', [
+            FTL.Message(
+                id=FTL.Identifier('dnt-description'),
+                value=COPY(
+                    'privacy.dtd',
+                    'doNotTrack.description'
+                )
+            ),
+
+            # Migrate an extra string to populate the iterator returned by
+            # ctx.merge_changeset(). Otherwise it won't yield anything if the
+            # merged contents are the same as the existing file.
+            FTL.Message(
+                id=FTL.Identifier('dnt-always'),
+                value=COPY(
+                    'privacy.dtd',
+                    'doNotTrack.always.label'
+                )
+            ),
+        ])
+
+        expected = {
+            'privacy.ftl': ftl_resource_to_json('''
+        // This Source Code Form is subject to the terms of the Mozilla Public
+        // License, v. 2.0. If a copy of the MPL was not distributed with this
+        // file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+        dnt-description = New Description in Polish
+        dnt-always = Zawsze
+            ''')
+        }
+
+        self.maxDiff = None
+        self.assertDictEqual(
+            to_json(self.ctx.merge_changeset()),
+            expected
+        )
+
+    def test_existing_target_ftl_with_all_messages(self):
+        self.ctx.add_transforms('privacy.ftl', 'privacy.ftl', [
+            FTL.Message(
+                id=FTL.Identifier('dnt-description'),
+                value=COPY(
+                    'privacy.dtd',
+                    'doNotTrack.description'
+                )
+            ),
+        ])
+
+        # All migrated messages are already in the target FTL and the result of
+        # merge_changeset is an empty iterator.
+        self.assertDictEqual(
+            to_json(self.ctx.merge_changeset()),
+            {}
+        )
+
+
+class TestNotSupportedError(unittest.TestCase):
+    def test_add_ftl(self):
+        pattern = ('Migrating translations from Fluent files is not supported')
+        with self.assertRaisesRegexp(NotSupportedError, pattern):
+            ctx = MergeContext(
+                lang='pl',
+                reference_dir=here('fixtures/en-US'),
+                localization_dir=here('fixtures/pl')
+            )
+
+            ctx.maybe_add_localization('privacy.ftl')
diff --git a/tests/migrate/test_context_real_examples.py b/tests/migrate/test_context_real_examples.py
index 7953885e..e62f47ed 100644
--- a/tests/migrate/test_context_real_examples.py
+++ b/tests/migrate/test_context_real_examples.py
@@ -27,14 +27,13 @@ def setUp(self):
             localization_dir=here('fixtures/pl')
         )
 
-        self.ctx.add_reference('aboutDownloads.ftl')
         try:
-            self.ctx.add_localization('aboutDownloads.dtd')
-            self.ctx.add_localization('aboutDownloads.properties')
+            self.ctx.maybe_add_localization('aboutDownloads.dtd')
+            self.ctx.maybe_add_localization('aboutDownloads.properties')
         except RuntimeError:
             self.skipTest('compare-locales required')
 
-        self.ctx.add_transforms('aboutDownloads.ftl', [
+        self.ctx.add_transforms('aboutDownloads.ftl', 'aboutDownloads.ftl', [
             FTL.Message(
                 id=FTL.Identifier('title'),
                 value=COPY(
@@ -289,12 +288,11 @@ def setUp(self):
         )
 
         try:
-            self.ctx.add_reference('aboutDialog.ftl')
-            self.ctx.add_localization('aboutDialog.dtd')
+            self.ctx.maybe_add_localization('aboutDialog.dtd')
         except RuntimeError:
             self.skipTest('compare-locales required')
 
-        self.ctx.add_transforms('aboutDialog.ftl', [
+        self.ctx.add_transforms('aboutDialog.ftl', 'aboutDialog.ftl', [
             FTL.Message(
                 id=FTL.Identifier('update-failed'),
                 value=CONCAT(
diff --git a/tests/migrate/test_copy.py b/tests/migrate/test_copy.py
index 55e78a20..66eb8c85 100644
--- a/tests/migrate/test_copy.py
+++ b/tests/migrate/test_copy.py
@@ -15,6 +15,8 @@
 
 class MockContext(unittest.TestCase):
     def get_source(self, path, key):
+        # Ignore path (test.properties) and get translations from self.strings
+        # defined in setUp.
         return self.strings.get(key, None).val
 
 
@@ -33,7 +35,7 @@ def setUp(self):
     def test_copy(self):
         msg = FTL.Message(
             FTL.Identifier('foo'),
-            value=COPY(self.strings, 'foo')
+            value=COPY('test.properties', 'foo')
         )
 
         self.assertEqual(
@@ -46,7 +48,7 @@ def test_copy(self):
     def test_copy_escape_unicode_middle(self):
         msg = FTL.Message(
             FTL.Identifier('foo-unicode-middle'),
-            value=COPY(self.strings, 'foo.unicode.middle')
+            value=COPY('test.properties', 'foo.unicode.middle')
         )
 
         self.assertEqual(
@@ -60,7 +62,7 @@ def test_copy_escape_unicode_middle(self):
     def test_copy_escape_unicode_begin(self):
         msg = FTL.Message(
             FTL.Identifier('foo-unicode-begin'),
-            value=COPY(self.strings, 'foo.unicode.begin')
+            value=COPY('test.properties', 'foo.unicode.begin')
         )
 
         self.assertEqual(
@@ -74,7 +76,7 @@ def test_copy_escape_unicode_begin(self):
     def test_copy_escape_unicode_end(self):
         msg = FTL.Message(
             FTL.Identifier('foo-unicode-end'),
-            value=COPY(self.strings, 'foo.unicode.end')
+            value=COPY('test.properties', 'foo.unicode.end')
         )
 
         self.assertEqual(
@@ -87,7 +89,7 @@ def test_copy_escape_unicode_end(self):
     def test_copy_html_entity(self):
         msg = FTL.Message(
             FTL.Identifier('foo-html-entity'),
-            value=COPY(self.strings, 'foo.html.entity')
+            value=COPY('test.properties', 'foo.html.entity')
         )
 
         self.assertEqual(
@@ -99,7 +101,7 @@ def test_copy_html_entity(self):
 
 
 @unittest.skipUnless(DTDParser, 'compare-locales required')
-class TestCopyTraits(MockContext):
+class TestCopyAttributes(MockContext):
     def setUp(self):
         self.strings = parse(DTDParser, '''
             <!ENTITY checkForUpdatesButton.label       "Check for updates">
@@ -112,12 +114,12 @@ def test_copy_accesskey(self):
             attributes=[
                 FTL.Attribute(
                     FTL.Identifier('label'),
-                    COPY(self.strings, 'checkForUpdatesButton.label')
+                    COPY('test.properties', 'checkForUpdatesButton.label')
                 ),
                 FTL.Attribute(
                     FTL.Identifier('accesskey'),
                     COPY(
-                        self.strings, 'checkForUpdatesButton.accesskey'
+                        'test.properties', 'checkForUpdatesButton.accesskey'
                     )
                 ),
             ]
diff --git a/tests/migrate/test_merge.py b/tests/migrate/test_merge.py
index 4da4af44..cdf9ed58 100644
--- a/tests/migrate/test_merge.py
+++ b/tests/migrate/test_merge.py
@@ -17,6 +17,8 @@
 
 class MockContext(unittest.TestCase):
     def get_source(self, path, key):
+        # Ignore path (test.properties) and get translations from
+        # self.ab_cd_legacy defined in setUp.
         return self.ab_cd_legacy.get(key, None).val
 
 
@@ -59,7 +61,7 @@ def setUp(self):
         self.transforms = [
             FTL.Message(
                 FTL.Identifier('title'),
-                value=COPY(None, 'aboutDownloads.title')
+                value=COPY('test.properties', 'aboutDownloads.title')
             ),
             FTL.Message(
                 FTL.Identifier('about'),
@@ -72,13 +74,13 @@ def setUp(self):
                 attributes=[
                     FTL.Attribute(
                         FTL.Identifier('label'),
-                        COPY(None, 'aboutDownloads.open')
+                        COPY('test.properties', 'aboutDownloads.open')
                     ),
                 ]
             ),
             FTL.Message(
                 FTL.Identifier('download-state-downloading'),
-                value=COPY(None, 'downloadState.downloading')
+                value=COPY('test.properties', 'downloadState.downloading')
             )
         ]
 
@@ -169,20 +171,20 @@ def setUp(self):
         self.transforms = [
             FTL.Message(
                 FTL.Identifier('title'),
-                value=COPY(None, 'aboutDownloads.title')
+                value=COPY('test.properties', 'aboutDownloads.title')
             ),
             FTL.Message(
                 FTL.Identifier('open-menuitem'),
                 attributes=[
                     FTL.Attribute(
                         FTL.Identifier('label'),
-                        COPY(None, 'aboutDownloads.open')
+                        COPY('test.properties', 'aboutDownloads.open')
                     ),
                 ]
             ),
             FTL.Message(
                 FTL.Identifier('download-state-downloading'),
-                value=COPY(None, 'downloadState.downloading')
+                value=COPY('test.properties', 'downloadState.downloading')
             )
         ]
 
@@ -282,11 +284,11 @@ def setUp(self):
         self.transforms = [
             FTL.Message(
                 FTL.Identifier('title'),
-                value=COPY(None, 'aboutDownloads.title')
+                value=COPY('test.properties', 'aboutDownloads.title')
             ),
             FTL.Message(
                 FTL.Identifier('download-state-downloading'),
-                value=COPY(None, 'downloadState.downloading')
+                value=COPY('test.properties', 'downloadState.downloading')
             )
         ]
 
diff --git a/tests/migrate/test_plural.py b/tests/migrate/test_plural.py
index 095f232d..0fb21726 100644
--- a/tests/migrate/test_plural.py
+++ b/tests/migrate/test_plural.py
@@ -19,6 +19,8 @@ class MockContext(unittest.TestCase):
     plural_categories = ('one', 'other')
 
     def get_source(self, path, key):
+        # Ignore path (test.properties) and get translations from self.strings
+        # defined in setUp.
         return self.strings.get(key, None).val
 
 
@@ -32,7 +34,7 @@ def setUp(self):
         self.message = FTL.Message(
             FTL.Identifier('delete-all'),
             value=PLURALS(
-                self.strings,
+                'test.properties',
                 'deleteAll',
                 EXTERNAL_ARGUMENT('num')
             )
@@ -86,7 +88,7 @@ def setUp(self):
         self.message = FTL.Message(
             FTL.Identifier('delete-all'),
             value=PLURALS(
-                self.strings,
+                'test.properties',
                 'deleteAll',
                 EXTERNAL_ARGUMENT('num')
             )
@@ -116,7 +118,7 @@ def test_plural_replace(self):
         msg = FTL.Message(
             FTL.Identifier('delete-all'),
             value=PLURALS(
-                self.strings,
+                'test.properties',
                 'deleteAll',
                 EXTERNAL_ARGUMENT('num'),
                 lambda text: REPLACE_IN_TEXT(
diff --git a/tests/migrate/test_replace.py b/tests/migrate/test_replace.py
index 1157daf4..65576d00 100644
--- a/tests/migrate/test_replace.py
+++ b/tests/migrate/test_replace.py
@@ -16,6 +16,8 @@
 
 class MockContext(unittest.TestCase):
     def get_source(self, path, key):
+        # Ignore path (test.properties) and get translations from self.strings
+        # defined in setUp.
         return self.strings.get(key, None).val
 
 
@@ -33,7 +35,7 @@ def test_replace_one(self):
         msg = FTL.Message(
             FTL.Identifier(u'hello'),
             value=REPLACE(
-                self.strings,
+                'test.properties',
                 'hello',
                 {
                     '#1': EXTERNAL_ARGUMENT('username')
@@ -52,7 +54,7 @@ def test_replace_two(self):
         msg = FTL.Message(
             FTL.Identifier(u'welcome'),
             value=REPLACE(
-                self.strings,
+                'test.properties',
                 'welcome',
                 {
                     '#1': EXTERNAL_ARGUMENT('username'),
@@ -72,7 +74,7 @@ def test_replace_too_many(self):
         msg = FTL.Message(
             FTL.Identifier(u'welcome'),
             value=REPLACE(
-                self.strings,
+                'test.properties',
                 'welcome',
                 {
                     '#1': EXTERNAL_ARGUMENT('username'),
@@ -93,7 +95,7 @@ def test_replace_too_few(self):
         msg = FTL.Message(
             FTL.Identifier(u'welcome'),
             value=REPLACE(
-                self.strings,
+                'test.properties',
                 'welcome',
                 {
                     '#1': EXTERNAL_ARGUMENT('username')
@@ -112,7 +114,7 @@ def test_replace_first(self):
         msg = FTL.Message(
             FTL.Identifier(u'first'),
             value=REPLACE(
-                self.strings,
+                'test.properties',
                 'first',
                 {
                     '#1': EXTERNAL_ARGUMENT('foo')
@@ -131,7 +133,7 @@ def test_replace_last(self):
         msg = FTL.Message(
             FTL.Identifier(u'last'),
             value=REPLACE(
-                self.strings,
+                'test.properties',
                 'last',
                 {
                     '#1': EXTERNAL_ARGUMENT('bar')
diff --git a/tests/migrate/test_source.py b/tests/migrate/test_source.py
new file mode 100644
index 00000000..1da96ac4
--- /dev/null
+++ b/tests/migrate/test_source.py
@@ -0,0 +1,51 @@
+# coding=utf8
+from __future__ import unicode_literals
+
+import unittest
+
+import fluent.syntax.ast as FTL
+
+from fluent.migrate.errors import NotSupportedError
+from fluent.migrate.transforms import Source, COPY, PLURALS, REPLACE
+from fluent.migrate.helpers import EXTERNAL_ARGUMENT
+
+
+class TestNotSupportedError(unittest.TestCase):
+    def test_source(self):
+        pattern = ('Migrating translations from Fluent files is not supported')
+        with self.assertRaisesRegexp(NotSupportedError, pattern):
+            Source('test.ftl', 'foo')
+
+    def test_copy(self):
+        pattern = ('Migrating translations from Fluent files is not supported')
+        with self.assertRaisesRegexp(NotSupportedError, pattern):
+            FTL.Message(
+                FTL.Identifier('foo'),
+                value=COPY('test.ftl', 'foo')
+            )
+
+    def test_plurals(self):
+        pattern = ('Migrating translations from Fluent files is not supported')
+        with self.assertRaisesRegexp(NotSupportedError, pattern):
+            FTL.Message(
+                FTL.Identifier('delete-all'),
+                value=PLURALS(
+                    'test.ftl',
+                    'deleteAll',
+                    EXTERNAL_ARGUMENT('num')
+                )
+            )
+
+    def test_replace(self):
+        pattern = ('Migrating translations from Fluent files is not supported')
+        with self.assertRaisesRegexp(NotSupportedError, pattern):
+            FTL.Message(
+                FTL.Identifier(u'hello'),
+                value=REPLACE(
+                    'test.ftl',
+                    'hello',
+                    {
+                        '#1': EXTERNAL_ARGUMENT('username')
+                    }
+                )
+            )
diff --git a/tools/migrate/examples/about_dialog.py b/tools/migrate/examples/about_dialog.py
index 84dae067..17ed610e 100644
--- a/tools/migrate/examples/about_dialog.py
+++ b/tools/migrate/examples/about_dialog.py
@@ -9,10 +9,9 @@
 def migrate(ctx):
     """Migrate about:dialog, part {index}"""
 
-    ctx.add_reference('browser/about_dialog.ftl', realpath='about_dialog.ftl')
-    ctx.add_localization('browser/chrome/browser/aboutDialog.dtd')
+    ctx.maybe_add_localization('browser/chrome/browser/aboutDialog.dtd')
 
-    ctx.add_transforms('browser/about_dialog.ftl', [
+    ctx.add_transforms('browser/about_dialog.ftl', 'about_dialog.ftl', [
         FTL.Message(
             id=FTL.Identifier('update-failed'),
             value=CONCAT(
diff --git a/tools/migrate/examples/about_downloads.py b/tools/migrate/examples/about_downloads.py
index 4d8ba1b2..35cd2beb 100644
--- a/tools/migrate/examples/about_downloads.py
+++ b/tools/migrate/examples/about_downloads.py
@@ -7,14 +7,12 @@
 def migrate(ctx):
     """Migrate about:download in Firefox for Android, part {index}"""
 
-    ctx.add_reference(
-        'mobile/about_downloads.ftl',
-        realpath='about_downloads.ftl'
-    )
-    ctx.add_localization('mobile/android/chrome/aboutDownloads.dtd')
-    ctx.add_localization('mobile/android/chrome/aboutDownloads.properties')
+    ctx.maybe_add_localization(
+        'mobile/android/chrome/aboutDownloads.dtd')
+    ctx.maybe_add_localization(
+        'mobile/android/chrome/aboutDownloads.properties')
 
-    ctx.add_transforms('mobile/about_downloads.ftl', [
+    ctx.add_transforms('mobile/about_downloads.ftl', 'about_downloads.ftl', [
         FTL.Message(
             id=FTL.Identifier('title'),
             value=COPY(
diff --git a/tools/migrate/examples/bug_1291693.py b/tools/migrate/examples/bug_1291693.py
index eebce64d..e745c9a5 100644
--- a/tools/migrate/examples/bug_1291693.py
+++ b/tools/migrate/examples/bug_1291693.py
@@ -7,25 +7,12 @@
 def migrate(ctx):
     """Bug 1291693 - Migrate the menubar to FTL, part {index}"""
 
-    ctx.add_reference(
-        'browser/menubar.ftl',
-        realpath='menubar.ftl'
-    )
-    ctx.add_reference(
-        'browser/toolbar.ftl',
-        realpath='toolbar.ftl'
-    )
-    ctx.add_reference(
-        'browser/branding/official/brand.ftl',
-        realpath='brand.ftl'
-    )
+    ctx.maybe_add_localization('browser/chrome/browser/browser.dtd')
+    ctx.maybe_add_localization('browser/chrome/browser/browser.properties')
+    ctx.maybe_add_localization('browser/branding/official/brand.dtd')
+    ctx.maybe_add_localization('browser/branding/official/brand.properties')
 
-    ctx.add_localization('browser/chrome/browser/browser.dtd')
-    ctx.add_localization('browser/chrome/browser/browser.properties')
-    ctx.add_localization('browser/branding/official/brand.dtd')
-    ctx.add_localization('browser/branding/official/brand.properties')
-
-    ctx.add_transforms('browser/menubar.ftl', [
+    ctx.add_transforms('browser/menubar.ftl', 'menubar.ftl', [
         FTL.Message(
             id=FTL.Identifier('file-menu'),
             attributes=[
@@ -1795,7 +1782,7 @@ def migrate(ctx):
         ),
     ])
 
-    ctx.add_transforms('browser/toolbar.ftl', [
+    ctx.add_transforms('browser/toolbar.ftl', 'toolbar.ftl', [
         FTL.Message(
             id=FTL.Identifier('urlbar-textbox'),
             attributes=[
@@ -1889,7 +1876,7 @@ def migrate(ctx):
         ),
     ])
 
-    ctx.add_transforms('browser/branding/official/brand.ftl', [
+    ctx.add_transforms('browser/branding/official/brand.ftl', 'brand.ftl', [
         FTL.Message(
             id=FTL.Identifier('brand-shorter-name'),
             value=COPY(
diff --git a/tools/migrate/migrate-l10n.py b/tools/migrate/migrate-l10n.py
index 6420991a..d8cf5cb1 100755
--- a/tools/migrate/migrate-l10n.py
+++ b/tools/migrate/migrate-l10n.py
@@ -2,14 +2,18 @@
 # coding=utf8
 
 import os
+import sys
 import json
+import logging
 import argparse
 import importlib
 
 import hglib
 from hglib.util import b
 
-from fluent.migrate import MergeContext, convert_blame_to_changesets
+from fluent.migrate import (
+    MergeContext, MigrationError, convert_blame_to_changesets
+)
 from blame import Blame
 
 
@@ -25,18 +29,20 @@ def main(lang, reference_dir, localization_dir, blame, migrations, dry_run):
         # For each migration create a new context.
         ctx = MergeContext(lang, reference_dir, localization_dir)
 
-        # Add the migration spec.
-        migration.migrate(ctx)
+        try:
+            # Add the migration spec.
+            migration.migrate(ctx)
+        except MigrationError as err:
+            sys.exit(err.message)
 
         # Keep track of how many changesets we're committing.
         index = 0
 
         for changeset in changesets:
-            # Run the migration.
+            # Run the migration for the changeset.
             snapshot = ctx.serialize_changeset(changeset['changes'])
 
-            # The current changeset didn't touch any of the translations
-            # affected by the migration.
+            # Did it change any files?
             if not snapshot:
                 continue
 
@@ -95,6 +101,9 @@ def main(lang, reference_dir, localization_dir, blame, migrations, dry_run):
     )
     parser.set_defaults(dry_run=False)
 
+    logger = logging.getLogger('migrate')
+    logger.setLevel(logging.INFO)
+
     args = parser.parse_args()
 
     if args.blame: