diff --git a/src/_includes/cwl/expressions/custom-functions.js b/src/_includes/cwl/expressions/custom-functions.js
new file mode 100644
index 00000000..6e9d05f7
--- /dev/null
+++ b/src/_includes/cwl/expressions/custom-functions.js
@@ -0,0 +1,18 @@
+/**
+ * Capitalize each word passed. Will split the text by spaces.
+ * For instance, given "hello world", it returns "Hello World".
+ *
+ * @param {String} message - The input message.
+ * @return {String} the message with each word with its initial letter capitalized.
+ */
+function capitalizeWords (message) {
+  if (message === undefined || message === null || typeof message !== 'string' || message.trim().length === 0) {
+    return '';
+  }
+  return message
+    .split(' ')
+    .map(function (token) {
+      return token.charAt(0).toUpperCase() + token.slice(1);
+    })
+    .join(' ');
+}
diff --git a/src/_includes/cwl/expressions/hello-world-expressionlib-external.cwl b/src/_includes/cwl/expressions/hello-world-expressionlib-external.cwl
new file mode 100644
index 00000000..9cc416af
--- /dev/null
+++ b/src/_includes/cwl/expressions/hello-world-expressionlib-external.cwl
@@ -0,0 +1,16 @@
+cwlVersion: v1.2
+class: CommandLineTool
+requirements:
+  - class: InlineJavascriptRequirement
+    expressionLib:
+      - { $include: custom-functions.js }
+
+baseCommand: echo
+
+inputs:
+  message:
+    type: string
+
+arguments: [$( capitalizeWords(inputs.message) )]
+
+outputs: []
diff --git a/src/_includes/cwl/expressions/hello-world-expressionlib-inline.cwl b/src/_includes/cwl/expressions/hello-world-expressionlib-inline.cwl
new file mode 100644
index 00000000..113b65f7
--- /dev/null
+++ b/src/_includes/cwl/expressions/hello-world-expressionlib-inline.cwl
@@ -0,0 +1,34 @@
+cwlVersion: v1.2
+class: CommandLineTool
+requirements:
+  - class: InlineJavascriptRequirement
+    expressionLib:
+      - |
+        /**
+         * Capitalize each word passed. Will split the text by spaces.
+         * For instance, given "hello world", it returns "Hello World".
+         *
+         * @param {String} message - The input message.
+         * @return {String} the message with each word with its initial letter capitalized.
+         */
+        function capitalizeWords (message) {
+          if (message === undefined || message === null || typeof message !== 'string' || message.trim().length === 0) {
+            return '';
+          }
+          return message
+            .split(' ')
+            .map(function (token) {
+              return token.charAt(0).toUpperCase() + token.slice(1);
+            })
+            .join(' ');
+        }
+
+baseCommand: echo
+
+inputs:
+  message:
+    type: string
+
+arguments: [$( capitalizeWords(inputs.message) )]
+
+outputs: []
diff --git a/src/_includes/cwl/expressions/hello-world-expressionlib.cwl b/src/_includes/cwl/expressions/hello-world-expressionlib.cwl
new file mode 100644
index 00000000..21ea73a2
--- /dev/null
+++ b/src/_includes/cwl/expressions/hello-world-expressionlib.cwl
@@ -0,0 +1,27 @@
+cwlVersion: v1.2
+class: CommandLineTool
+requirements:
+  - class: InlineJavascriptRequirement
+    expressionLib:
+      - { $include: custom-functions.js }
+      - |
+        /**
+         * A merely illustrative example function that uses a function
+         * from the included custom-functions.js file to create a
+         * Hello World message.
+         *
+         * @param {Object} message - CWL document input message
+         */
+        var createHelloWorldMessage = function (message) {
+          return capitalizeWords(message);
+        };
+
+baseCommand: echo
+
+inputs:
+  message:
+    type: string
+
+arguments: [$( createHelloWorldMessage(inputs.message) )]
+
+outputs: []
diff --git a/src/topics/expressions.md b/src/topics/expressions.md
index 31b3ee9e..eebc027b 100644
--- a/src/topics/expressions.md
+++ b/src/topics/expressions.md
@@ -11,6 +11,7 @@ When manipulating file names, extensions, paths etc, consider whether one of the
 [built in `File` properties][file-prop] like `basename`, `nameroot`, `nameext`,
 etc, could be used instead.
 See the [list of best practices](best-practices.md).
+```
 
 ```{literalinclude} /_includes/cwl/expressions/expression.cwl
 :language: cwl
@@ -112,7 +113,91 @@ only in certain fields.  These are:
 
 [file-prop]: https://www.commonwl.org/v1.0/CommandLineTool.html#File
 
+## Using external libraries and inline JavaScript code with `expressionLib`
+
+The requirement `InlineJavascriptRequirement` supports an `expressionLib` attribute
+that allows users to load external JavaScript files, or to provide inline JavaScript
+code.
+
+Entries added to the `expressionLib` attribute are parsed with the JavaScript engine
+of a CWL runner. This can be used to include external files or to create JavaScript
+functions that can be called in other parts of the CWL document.
+
+```{note}
+
+The CWL standards (versions 1.0 through 1.2) [states](https://www.commonwl.org/v1.0/CommandLineTool.html#Expressions)
+ that the only version of JavaScript valid in CWL expressions is
+[ECMAScript 5.1](https://262.ecma-international.org/5.1/). This means that any
+code that you include or write in your CWL Document must be compliant with
+ECMAScript 5.1.
+```
+
+For example, we can use `InlineJavascriptRequirement` and write a JavaScript function
+inline in `expressionLib`. That function can then be used in other parts of the
+CWL document:
+
+```{literalinclude} /_includes/cwl/expressions/hello-world-expressionlib-inline.cwl
+:language: cwl
+:caption: "`hello-world-expressionlib-inline.cwl`"
+:name: "`hello-world-expressionlib-inline.cwl`"
+:emphasize-lines: 5, 14, 32
+```
+
+Running this CWL workflow will invoke the JavaScript function and result in
+the `echo` command printing the input message with capital initial letters:
+
+```{runcmd} cwltool hello-world-expressionlib-inline.cwl --message "hello world"
+:caption: "Running `hello-world-expressionlib-inline.cwl`."
+:name: running-hell-world-expressionlib-inline-cwl
+:working-directory: src/_includes/cwl/expressions/
+```
+
+Let's move the `capitalizeWords` function to an external file, `custom-functions.js`, and
+import it in our CWL document:
+
+```{literalinclude} /_includes/cwl/expressions/custom-functions.js
+:language: javascript
+:caption: "`custom-functions.js`"
+:name: "`custom-functions.js`"
+```
+
+```{literalinclude} /_includes/cwl/expressions/hello-world-expressionlib-external.cwl
+:language: cwl
+:caption: "`hello-world-expressionlib-external.cwl`"
+:name: "`hello-world-expressionlib-external.cwl`"
+:emphasize-lines: 5-6, 14
+```
+
+The `custom-functions.js` file is included in the CWL document with the `$include: custom-functions.js`
+statement. That makes the functions and variables available to be used in other parts of
+the CWL document.
+
+```{runcmd} cwltool hello-world-expressionlib-external.cwl --message "hello world"
+:caption: "Running `hello-world-expressionlib-external.cwl`."
+:name: running-hell-world-expressionlib-external-cwl
+:working-directory: src/_includes/cwl/expressions/
+```
 
-% TODO
-% - (maybe not before other concepts? move this to after inputs/outputs/etc?)
-% - External libraries and expressionLib - https://github.com/common-workflow-language/user_guide/issues/126
+Finally, note that you can have both inline and external JavaScript code in your
+CWL document. In this final example we have added another entry to the `expressionLib`
+attribute with the new function `createHelloWorldMessage`, that calls the `capitalizeWords`
+function from the external file `custom-functions.js`.
+
+```{literalinclude} /_includes/cwl/expressions/hello-world-expressionlib.cwl
+:language: cwl
+:caption: "`hello-world-expressionlib.cwl`"
+:name: "`hello-world-expressionlib.cwl`"
+:emphasize-lines: 5-17, 25
+```
+
+```{runcmd} cwltool hello-world-expressionlib.cwl --message "hello world"
+:caption: "Running `hello-world-expressionlib.cwl`."
+:name: running-hell-world-expressionlib-cwl
+:working-directory: src/_includes/cwl/expressions/
+```
+
+```{note}
+The `$include` statement can be used to include a file from the local disk or from a remote location.
+It works with both relative and absolute paths. Read the [text about `$include`](https://www.commonwl.org/v1.0/SchemaSalad.html#Include)
+from the CWL specification to learn more about it.
+```