diff --git a/GoogleDataTransport/GoogleDataTransport/Classes/GDTReachability.h b/GoogleDataTransport/GoogleDataTransport/Classes/GDTReachability.h
new file mode 100644
index 00000000000..27f0feb808a
--- /dev/null
+++ b/GoogleDataTransport/GoogleDataTransport/Classes/GDTReachability.h
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2019 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import <SystemConfiguration/SCNetworkReachability.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/** This class helps determine upload conditions by determining connectivity. */
+@interface GDTReachability : NSObject
+
+/** The current set flags indicating network conditions */
++ (SCNetworkReachabilityFlags)currentFlags;
+
+@end
+
+NS_ASSUME_NONNULL_END
diff --git a/GoogleDataTransport/GoogleDataTransport/Classes/GDTReachability.m b/GoogleDataTransport/GoogleDataTransport/Classes/GDTReachability.m
new file mode 100644
index 00000000000..baedc4b6b72
--- /dev/null
+++ b/GoogleDataTransport/GoogleDataTransport/Classes/GDTReachability.m
@@ -0,0 +1,104 @@
+/*
+ * Copyright 2019 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import "GDTReachability.h"
+#import "GDTReachability_Private.h"
+
+#import <netinet/in.h>
+
+/** Sets the _callbackFlag ivar whenever the network changes.
+ *
+ * @param reachability The reachability object calling back.
+ * @param flags The new flag values.
+ * @param info Any data that might be passed in by the callback.
+ */
+static void GDTReachabilityCallback(SCNetworkReachabilityRef reachability,
+                                    SCNetworkReachabilityFlags flags,
+                                    void *info);
+
+@implementation GDTReachability {
+  /** The reachability object. */
+  SCNetworkReachabilityRef _reachabilityRef;
+
+  /** The queue on which callbacks and all work will occur. */
+  dispatch_queue_t _reachabilityQueue;
+
+  /** Flags specified by reachability callbacks. */
+  SCNetworkConnectionFlags _callbackFlags;
+}
+
++ (void)load {
+  [self sharedInstance];
+}
+
++ (instancetype)sharedInstance {
+  static GDTReachability *sharedInstance;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    sharedInstance = [[GDTReachability alloc] init];
+  });
+  return sharedInstance;
+}
+
++ (SCNetworkReachabilityFlags)currentFlags {
+  __block SCNetworkReachabilityFlags currentFlags;
+  dispatch_sync([GDTReachability sharedInstance] -> _reachabilityQueue, ^{
+    GDTReachability *reachability = [GDTReachability sharedInstance];
+    currentFlags = reachability->_flags ? reachability->_flags : reachability->_callbackFlags;
+  });
+  return currentFlags;
+}
+
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    struct sockaddr_in zeroAddress;
+    bzero(&zeroAddress, sizeof(zeroAddress));
+    zeroAddress.sin_len = sizeof(zeroAddress);
+    zeroAddress.sin_family = AF_INET;
+
+    _reachabilityQueue = dispatch_queue_create("com.google.GDTReachability", DISPATCH_QUEUE_SERIAL);
+    _reachabilityRef = SCNetworkReachabilityCreateWithAddress(
+        kCFAllocatorDefault, (const struct sockaddr *)&zeroAddress);
+    Boolean success = SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, _reachabilityQueue);
+    NSAssert(success, @"The reachability queue wasn't set.");
+    success = SCNetworkReachabilitySetCallback(_reachabilityRef, GDTReachabilityCallback, NULL);
+    NSAssert(success, @"The reachability callback wasn't set.");
+
+    // Get the initial set of flags.
+    dispatch_async(_reachabilityQueue, ^{
+      Boolean valid = SCNetworkReachabilityGetFlags(self->_reachabilityRef, &self->_flags);
+      if (!valid) {
+        self->_flags = 0;
+      }
+    });
+  }
+  return self;
+}
+
+- (void)setCallbackFlags:(SCNetworkReachabilityFlags)flags {
+  if (_callbackFlags != flags) {
+    self->_callbackFlags = flags;
+  }
+}
+
+@end
+
+static void GDTReachabilityCallback(SCNetworkReachabilityRef reachability,
+                                    SCNetworkReachabilityFlags flags,
+                                    void *info) {
+  [[GDTReachability sharedInstance] setCallbackFlags:flags];
+}
diff --git a/GoogleDataTransport/GoogleDataTransport/Classes/GDTUploadCoordinator.m b/GoogleDataTransport/GoogleDataTransport/Classes/GDTUploadCoordinator.m
index 5820444c63e..5ba819f82b1 100644
--- a/GoogleDataTransport/GoogleDataTransport/Classes/GDTUploadCoordinator.m
+++ b/GoogleDataTransport/GoogleDataTransport/Classes/GDTUploadCoordinator.m
@@ -20,6 +20,7 @@
 #import "GDTAssert.h"
 #import "GDTClock.h"
 #import "GDTConsoleLogger.h"
+#import "GDTReachability.h"
 #import "GDTRegistrar_Private.h"
 #import "GDTStorage.h"
 #import "GDTUploadPackage_Private.h"
@@ -85,8 +86,11 @@ - (void)forceUploadForTarget:(GDTTarget)target {
     if (self->_runningInBackground) {
       [self->_forcedUploadQueue insertObject:forceUploadBlock atIndex:0];
       [NSKeyedArchiver archiveRootObject:self toFile:[GDTUploadCoordinator archivePath]];
-
-      // Enqueue the force upload block if there's an in-flight upload for that target already.
+      // Enqueue the force upload block if conditions are bad or if there's an in-flight upload for
+      // that target already.
+    } else if (([self uploadConditions] & GDTUploadConditionNoNetwork) ==
+               GDTUploadConditionNoNetwork) {
+      [self->_forcedUploadQueue insertObject:forceUploadBlock atIndex:0];
     } else if (self->_targetToInFlightEventSet[targetNumber]) {
       [self->_forcedUploadQueue insertObject:forceUploadBlock atIndex:0];
     } else {
@@ -109,39 +113,34 @@ - (GDTStorage *)storage {
 // This should always be called in a thread-safe manner. When running the background, in theory,
 // the uploader's background task should be calling this.
 - (GDTUploaderCompletionBlock)onCompleteBlock {
-  __weak GDTUploadCoordinator *weakSelf = self;
   static GDTUploaderCompletionBlock onCompleteBlock;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
     onCompleteBlock = ^(GDTTarget target, GDTClock *nextUploadAttemptUTC, NSError *error) {
-      GDTUploadCoordinator *strongSelf = weakSelf;
-      if (strongSelf) {
-        dispatch_async(strongSelf.coordinationQueue, ^{
-          NSNumber *targetNumber = @(target);
-          if (error) {
-            GDTLogWarning(GDTMCWUploadFailed, @"Error during upload: %@", error);
-            [strongSelf->_targetToInFlightEventSet removeObjectForKey:targetNumber];
-            return;
-          }
-          strongSelf->_targetToNextUploadTimes[targetNumber] = nextUploadAttemptUTC;
-          NSSet<GDTStoredEvent *> *events =
-              [strongSelf->_targetToInFlightEventSet objectForKey:targetNumber];
-          GDTAssert(events, @"There should be an in-flight event set to remove.");
-          [strongSelf.storage removeEvents:events];
-          [strongSelf->_targetToInFlightEventSet removeObjectForKey:targetNumber];
-          if (strongSelf->_runningInBackground) {
-            [NSKeyedArchiver archiveRootObject:self toFile:[GDTUploadCoordinator archivePath]];
-          } else if (strongSelf->_forcedUploadQueue.count) {
-            GDTUploadCoordinatorForceUploadBlock queuedBlock =
-                [strongSelf->_forcedUploadQueue lastObject];
-            if (queuedBlock) {
-              dispatch_async(strongSelf->_coordinationQueue, ^{
-                queuedBlock();
-              });
-            }
+      dispatch_async(self->_coordinationQueue, ^{
+        NSNumber *targetNumber = @(target);
+        if (error) {
+          GDTLogWarning(GDTMCWUploadFailed, @"Error during upload: %@", error);
+          [self->_targetToInFlightEventSet removeObjectForKey:targetNumber];
+          return;
+        }
+        self->_targetToNextUploadTimes[targetNumber] = nextUploadAttemptUTC;
+        NSSet<GDTStoredEvent *> *events =
+            [self->_targetToInFlightEventSet objectForKey:targetNumber];
+        GDTAssert(events, @"There should be an in-flight event set to remove.");
+        [self->_storage removeEvents:events];
+        [self->_targetToInFlightEventSet removeObjectForKey:targetNumber];
+        if (self->_runningInBackground) {
+          [NSKeyedArchiver archiveRootObject:self toFile:[GDTUploadCoordinator archivePath]];
+        } else if (self->_forcedUploadQueue.count) {
+          GDTUploadCoordinatorForceUploadBlock queuedBlock = [self->_forcedUploadQueue lastObject];
+          if (queuedBlock) {
+            dispatch_async(self->_coordinationQueue, ^{
+              queuedBlock();
+            });
           }
-        });
-      }
+        }
+      });
     };
   });
   return onCompleteBlock;
@@ -153,20 +152,17 @@ - (GDTUploaderCompletionBlock)onCompleteBlock {
  * check the next-upload clocks of all targets to determine if an upload attempt can be made.
  */
 - (void)startTimer {
-  __weak GDTUploadCoordinator *weakSelf = self;
   dispatch_sync(_coordinationQueue, ^{
-    GDTUploadCoordinator *strongSelf = weakSelf;
-    GDTAssert(strongSelf, @"self must be real to start a timer.");
-    strongSelf->_timer =
-        dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, strongSelf->_coordinationQueue);
-    dispatch_source_set_timer(strongSelf->_timer, DISPATCH_TIME_NOW, strongSelf->_timerInterval,
-                              strongSelf->_timerLeeway);
-    dispatch_source_set_event_handler(strongSelf->_timer, ^{
-      if (!strongSelf->_runningInBackground) {
-        [strongSelf checkPrioritizersAndUploadEvents];
+    self->_timer =
+        dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, self->_coordinationQueue);
+    dispatch_source_set_timer(self->_timer, DISPATCH_TIME_NOW, self->_timerInterval,
+                              self->_timerLeeway);
+    dispatch_source_set_event_handler(self->_timer, ^{
+      if (!self->_runningInBackground) {
+        [self checkPrioritizersAndUploadEvents];
       }
     });
-    dispatch_resume(strongSelf->_timer);
+    dispatch_resume(self->_timer);
   });
 }
 
@@ -181,36 +177,72 @@ - (void)stopTimer {
  * events for that target or not. If so, queries the prioritizers
  */
 - (void)checkPrioritizersAndUploadEvents {
-  __weak GDTUploadCoordinator *weakSelf = self;
   dispatch_async(_coordinationQueue, ^{
     if (self->_runningInBackground) {
       return;
     }
+
+    GDTUploadConditions conds = [self uploadConditions];
+    if ((conds & GDTUploadConditionNoNetwork) == GDTUploadConditionNoNetwork) {
+      return;
+    }
+
     static int count = 0;
     count++;
-    GDTUploadCoordinator *strongSelf = weakSelf;
-    if (strongSelf) {
-      NSArray<NSNumber *> *targetsReadyForUpload = [self targetsReadyForUpload];
-      for (NSNumber *target in targetsReadyForUpload) {
-        id<GDTPrioritizer> prioritizer = strongSelf->_registrar.targetToPrioritizer[target];
-        id<GDTUploader> uploader = strongSelf->_registrar.targetToUploader[target];
-        GDTAssert(prioritizer && uploader, @"Target '%@' is missing an implementation", target);
-        GDTUploadConditions conds = [self uploadConditions];
-        GDTUploadPackage *package = [[prioritizer uploadPackageWithConditions:conds] copy];
-        package.storage = strongSelf.storage;
-        if (package.events && package.events.count > 0) {
-          strongSelf->_targetToInFlightEventSet[target] = package.events;
-          [uploader uploadPackage:package onComplete:self.onCompleteBlock];
-        }
+    NSArray<NSNumber *> *targetsReadyForUpload = [self targetsReadyForUpload];
+    for (NSNumber *target in targetsReadyForUpload) {
+      id<GDTPrioritizer> prioritizer = self->_registrar.targetToPrioritizer[target];
+      id<GDTUploader> uploader = self->_registrar.targetToUploader[target];
+      GDTAssert(prioritizer && uploader, @"Target '%@' is missing an implementation", target);
+      GDTUploadPackage *package = [[prioritizer uploadPackageWithConditions:conds] copy];
+      package.storage = self.storage;
+      if (package.events && package.events.count > 0) {
+        self->_targetToInFlightEventSet[target] = package.events;
+        [uploader uploadPackage:package onComplete:self.onCompleteBlock];
       }
     }
   });
 }
 
-/** */
+/** Returns the current upload conditions after making determinations about the network connection.
+ *
+ * @return The current upload conditions.
+ */
 - (GDTUploadConditions)uploadConditions {
-  // TODO: Compute the real upload conditions.
-  return GDTUploadConditionMobileData;
+  SCNetworkReachabilityFlags currentFlags = [GDTReachability currentFlags];
+
+  BOOL reachable =
+      (currentFlags & kSCNetworkReachabilityFlagsReachable) == kSCNetworkReachabilityFlagsReachable;
+  BOOL connectionRequired = (currentFlags & kSCNetworkReachabilityFlagsConnectionRequired) ==
+                            kSCNetworkReachabilityFlagsConnectionRequired;
+  BOOL interventionRequired = (currentFlags & kSCNetworkReachabilityFlagsInterventionRequired) ==
+                              kSCNetworkReachabilityFlagsInterventionRequired;
+  BOOL connectionOnDemand = (currentFlags & kSCNetworkReachabilityFlagsConnectionOnDemand) ==
+                            kSCNetworkReachabilityFlagsConnectionOnDemand;
+  BOOL connectionOnTraffic = (currentFlags & kSCNetworkReachabilityFlagsConnectionOnTraffic) ==
+                             kSCNetworkReachabilityFlagsConnectionOnTraffic;
+  BOOL isWWAN =
+      (currentFlags & kSCNetworkReachabilityFlagsIsWWAN) == kSCNetworkReachabilityFlagsIsWWAN;
+
+  if (!reachable) {
+    return GDTUploadConditionNoNetwork;
+  }
+
+  GDTUploadConditions conditions = 0;
+  conditions |= !connectionRequired ? GDTUploadConditionWifiData : conditions;
+  conditions |= isWWAN ? GDTUploadConditionMobileData : conditions;
+  if ((connectionOnTraffic || connectionOnDemand) && !interventionRequired) {
+    conditions = GDTUploadConditionWifiData;
+  }
+
+  BOOL wifi = (conditions & GDTUploadConditionWifiData) == GDTUploadConditionWifiData;
+  BOOL cell = (conditions & GDTUploadConditionMobileData) == GDTUploadConditionMobileData;
+
+  if (!(wifi || cell)) {
+    conditions = GDTUploadConditionUnclearConnection;
+  }
+
+  return conditions;
 }
 
 /** Checks the next upload time for each target and returns an array of targets that are
diff --git a/GoogleDataTransport/GoogleDataTransport/Classes/Private/GDTReachability_Private.h b/GoogleDataTransport/GoogleDataTransport/Classes/Private/GDTReachability_Private.h
new file mode 100644
index 00000000000..29c49706ff9
--- /dev/null
+++ b/GoogleDataTransport/GoogleDataTransport/Classes/Private/GDTReachability_Private.h
@@ -0,0 +1,30 @@
+/*
+ * Copyright 2019 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import "GDTReachability.h"
+
+@interface GDTReachability ()
+
+/** Allows manually setting the flags for testing purposes. */
+@property(nonatomic, readwrite) SCNetworkReachabilityFlags flags;
+
+/** Creates/returns the singleton instance of this class.
+ *
+ * @return The singleton instance of this class.
+ */
++ (instancetype)sharedInstance;
+
+@end
diff --git a/GoogleDataTransport/GoogleDataTransport/Classes/Public/GDTPrioritizer.h b/GoogleDataTransport/GoogleDataTransport/Classes/Public/GDTPrioritizer.h
index b9243f0a92d..23518926777 100644
--- a/GoogleDataTransport/GoogleDataTransport/Classes/Public/GDTPrioritizer.h
+++ b/GoogleDataTransport/GoogleDataTransport/Classes/Public/GDTPrioritizer.h
@@ -28,14 +28,20 @@ NS_ASSUME_NONNULL_BEGIN
  */
 typedef NS_OPTIONS(NSInteger, GDTUploadConditions) {
 
+  /** An upload shouldn't be attempted, because there's no network. */
+  GDTUploadConditionNoNetwork = 1 << 0,
+
   /** An upload would likely use mobile data. */
-  GDTUploadConditionMobileData,
+  GDTUploadConditionMobileData = 1 << 1,
 
   /** An upload would likely use wifi data. */
-  GDTUploadConditionWifiData,
+  GDTUploadConditionWifiData = 1 << 2,
+
+  /** An upload uses some sort of network connection, but it's unclear which. */
+  GDTUploadConditionUnclearConnection = 1 << 3,
 
   /** A high priority event has occurred. */
-  GDTUploadConditionHighPriority,
+  GDTUploadConditionHighPriority = 1 << 4,
 };
 
 /** This protocol defines the common interface of event prioritization. Prioritizers are
diff --git a/GoogleDataTransport/Tests/Integration/GDTIntegrationTest.m b/GoogleDataTransport/Tests/Integration/GDTIntegrationTest.m
index 04051f5188d..4086632b4b5 100644
--- a/GoogleDataTransport/Tests/Integration/GDTIntegrationTest.m
+++ b/GoogleDataTransport/Tests/Integration/GDTIntegrationTest.m
@@ -22,6 +22,7 @@
 #import "GDTIntegrationTestUploader.h"
 #import "GDTTestServer.h"
 
+#import "GDTReachability_Private.h"
 #import "GDTStorage_Private.h"
 #import "GDTUploadCoordinator+Testing.h"
 
@@ -84,6 +85,9 @@ - (void)testEndToEndEvent {
   XCTestExpectation *expectation = [self expectationWithDescription:@"server got the request"];
   expectation.assertForOverFulfill = NO;
 
+  // Manually set the reachability flag.
+  [GDTReachability sharedInstance].flags = kSCNetworkReachabilityFlagsReachable;
+
   // Create the server.
   GDTTestServer *testServer = [[GDTTestServer alloc] init];
   [testServer setResponseCompletedBlock:^(GCDWebServerRequest *_Nonnull request,
diff --git a/GoogleDataTransport/Tests/Integration/Helpers/GDTIntegrationTestPrioritizer.m b/GoogleDataTransport/Tests/Integration/Helpers/GDTIntegrationTestPrioritizer.m
index 07d4b4c260b..0e746b475ec 100644
--- a/GoogleDataTransport/Tests/Integration/Helpers/GDTIntegrationTestPrioritizer.m
+++ b/GoogleDataTransport/Tests/Integration/Helpers/GDTIntegrationTestPrioritizer.m
@@ -69,7 +69,7 @@ - (GDTUploadPackage *)uploadPackageWithConditions:(GDTUploadConditions)condition
       [[GDTIntegrationTestUploadPackage alloc] init];
   dispatch_sync(_queue, ^{
     if ((conditions & GDTUploadConditionWifiData) == GDTUploadConditionWifiData) {
-      uploadPackage.events = self.wifiOnlyEvents;
+      uploadPackage.events = [self.wifiOnlyEvents setByAddingObjectsFromSet:self.nonWifiEvents];
     } else {
       uploadPackage.events = self.nonWifiEvents;
     }
diff --git a/GoogleDataTransport/Tests/Unit/GDTTestCase.m b/GoogleDataTransport/Tests/Unit/GDTTestCase.m
index 5c788a80614..98b0bb1a1c3 100644
--- a/GoogleDataTransport/Tests/Unit/GDTTestCase.m
+++ b/GoogleDataTransport/Tests/Unit/GDTTestCase.m
@@ -16,11 +16,13 @@
 
 #import "GDTTestCase.h"
 
+#import "GDTReachability_Private.h"
 #import "GDTUploadCoordinator+Testing.h"
 
 @implementation GDTTestCase
 
 - (void)setUp {
+  [GDTReachability sharedInstance].flags = kSCNetworkReachabilityFlagsReachable;
   [[GDTUploadCoordinator sharedInstance] stopTimer];
 }
 
diff --git a/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTPrioritizer.m b/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTPrioritizer.m
index 7125173ba4d..b262aafca8f 100644
--- a/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTPrioritizer.m
+++ b/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTPrioritizer.m
@@ -168,4 +168,15 @@ typedef NS_ENUM(NSInteger, GDTCCTQoSTier) {
       }];
 }
 
+#pragma mark - GDTLifecycleProtocol
+
+- (void)appWillBackground:(UIApplication *)app {
+}
+
+- (void)appWillForeground:(UIApplication *)app {
+}
+
+- (void)appWillTerminate:(UIApplication *)application {
+}
+
 @end
diff --git a/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTUploader.m b/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTUploader.m
index 175befe572b..4ab61c732d1 100644
--- a/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTUploader.m
+++ b/GoogleDataTransportCCTSupport/GoogleDataTransportCCTSupport/Classes/GDTCCTUploader.m
@@ -138,4 +138,15 @@ - (nonnull NSData *)constructRequestProtoFromPackage:(GDTUploadPackage *)package
   return data ? data : [[NSData alloc] init];
 }
 
+#pragma mark - GDTLifecycleProtocol
+
+- (void)appWillBackground:(UIApplication *)app {
+}
+
+- (void)appWillForeground:(UIApplication *)app {
+}
+
+- (void)appWillTerminate:(UIApplication *)application {
+}
+
 @end