diff --git a/core/connection.js b/core/connection.js
index 81489c570..9da7b0171 100644
--- a/core/connection.js
+++ b/core/connection.js
@@ -200,6 +200,8 @@ Blockly.Connection.prototype.checkConnection_ = function(target) {
       throw 'Source connection already connected.';
     case Blockly.Connection.REASON_TARGET_NULL:
       throw 'Target connection is null.';
+    case Blockly.Connection.REASON_CHECKS_FAILED:
+      throw 'Connection checks failed.';
     default:
       throw 'Unknown connection failure: this should never happen!';
   }
@@ -293,8 +295,7 @@ Blockly.Connection.prototype.connect = function(otherConnection) {
   }
 
   // Establish the connections.
-  this.targetConnection = otherConnection;
-  otherConnection.targetConnection = this;
+  Blockly.Connection.connectReciprocally(this, otherConnection);
 
   // Demote the inferior block so that one is a child of the superior one.
   childBlock.setParent(parentBlock);
@@ -319,6 +320,19 @@ Blockly.Connection.prototype.connect = function(otherConnection) {
   }
 };
 
+/**
+ * Update two connections to target each other.
+ * @param {Blockly.Connection} first The first connection to update.
+ * @param {Blockly.Connection} second The second conneciton to update.
+ */
+Blockly.Connection.connectReciprocally = function(first, second) {
+  if (!first || !second) {
+    throw 'Cannot connect null connections.';
+  }
+  first.targetConnection = second;
+  second.targetConnection = first;
+}
+
 /**
  * Does the given block have one and only one connection point that will accept
  * an orphaned block?
diff --git a/tests/jsunit/connection_db_test.js b/tests/jsunit/connection_db_test.js
new file mode 100644
index 000000000..c7f897168
--- /dev/null
+++ b/tests/jsunit/connection_db_test.js
@@ -0,0 +1,95 @@
+/**
+ * @license
+ * Blockly Tests
+ *
+ * Copyright 2015 Google Inc.
+ * https://developers.google.com/blockly/
+ *
+ * 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.
+ */
+'use strict';
+
+function verify_DB_(msg, expected, db) {
+   var equal = (expected.length == db.length);
+   if (equal) {
+     for (var x = 0; x < expected.length; x++) {
+       if (expected[x] != db[x]) {
+         equal = false;
+         break;
+       }
+     }
+   }
+   if (equal) {
+     assertTrue(msg, true);
+   } else {
+     assertEquals(msg, expected, db);
+   }
+}
+
+function test_DB_addConnection() {
+  var db = new Blockly.ConnectionDB();
+  var o2 = {y_: 2, sourceBlock_: {}};
+  db.addConnection_(o2);
+  verify_DB_('Adding connection #2', [o2], db);
+
+  var o4 = {y_: 4, sourceBlock_: {}};
+  db.addConnection_(o4);
+  verify_DB_('Adding connection #4', [o2, o4], db);
+
+  var o1 = {y_: 1, sourceBlock_: {}};
+  db.addConnection_(o1);
+  verify_DB_('Adding connection #1', [o1, o2, o4], db);
+
+  var o3a = {y_: 3, sourceBlock_: {}};
+  db.addConnection_(o3a);
+  verify_DB_('Adding connection #3a', [o1, o2, o3a, o4], db);
+
+  var o3b = {y_: 3, sourceBlock_: {}};
+  db.addConnection_(o3b);
+  verify_DB_('Adding connection #3b', [o1, o2, o3b, o3a, o4], db);
+}
+
+function test_DB_removeConnection() {
+  var db = new Blockly.ConnectionDB();
+  var o1 = {y_: 1, sourceBlock_: {}};
+  var o2 = {y_: 2, sourceBlock_: {}};
+  var o3a = {y_: 3, sourceBlock_: {}};
+  var o3b = {y_: 3, sourceBlock_: {}};
+  var o3c = {y_: 3, sourceBlock_: {}};
+  var o4 = {y_: 4, sourceBlock_: {}};
+  db.addConnection_(o1);
+  db.addConnection_(o2);
+  db.addConnection_(o3c);
+  db.addConnection_(o3b);
+  db.addConnection_(o3a);
+  db.addConnection_(o4);
+  verify_DB_('Adding connections 1-4', [o1, o2, o3a, o3b, o3c, o4], db);
+
+  db.removeConnection_(o2);
+  verify_DB_('Removing connection #2', [o1, o3a, o3b, o3c, o4], db);
+
+  db.removeConnection_(o4);
+  verify_DB_('Removing connection #4', [o1, o3a, o3b, o3c], db);
+
+  db.removeConnection_(o1);
+  verify_DB_('Removing connection #1', [o3a, o3b, o3c], db);
+
+  db.removeConnection_(o3a);
+  verify_DB_('Removing connection #3a', [o3b, o3c], db);
+
+  db.removeConnection_(o3c);
+  verify_DB_('Removing connection #3c', [o3b], db);
+
+  db.removeConnection_(o3b);
+  verify_DB_('Removing connection #3b', [], db);
+}
diff --git a/tests/jsunit/connection_test.js b/tests/jsunit/connection_test.js
index c7f897168..b9bcc2b23 100644
--- a/tests/jsunit/connection_test.js
+++ b/tests/jsunit/connection_test.js
@@ -2,7 +2,7 @@
  * @license
  * Blockly Tests
  *
- * Copyright 2015 Google Inc.
+ * Copyright 2016 Google Inc.
  * https://developers.google.com/blockly/
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,79 +17,217 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+/**
+ * @fileoverview Tests for connection logic.
+ * @author fenichel@google.com (Rachel Fenichel)
+ */
 'use strict';
 
-function verify_DB_(msg, expected, db) {
-   var equal = (expected.length == db.length);
-   if (equal) {
-     for (var x = 0; x < expected.length; x++) {
-       if (expected[x] != db[x]) {
-         equal = false;
-         break;
-       }
-     }
-   }
-   if (equal) {
-     assertTrue(msg, true);
-   } else {
-     assertEquals(msg, expected, db);
-   }
+var input;
+var output;
+var previous;
+var next;
+
+var dummyWorkspace;
+
+function connectionTest_setUp() {
+  dummyWorkspace = {};
+  input = new Blockly.Connection({workspace: dummyWorkspace}, Blockly.INPUT_VALUE);
+  output = new Blockly.Connection({workspace: dummyWorkspace}, Blockly.OUTPUT_VALUE);
+  previous = new Blockly.Connection({workspace: dummyWorkspace}, Blockly.PREVIOUS_STATEMENT);
+  next = new Blockly.Connection({workspace: dummyWorkspace}, Blockly.NEXT_STATEMENT);
 }
 
-function test_DB_addConnection() {
-  var db = new Blockly.ConnectionDB();
-  var o2 = {y_: 2, sourceBlock_: {}};
-  db.addConnection_(o2);
-  verify_DB_('Adding connection #2', [o2], db);
-
-  var o4 = {y_: 4, sourceBlock_: {}};
-  db.addConnection_(o4);
-  verify_DB_('Adding connection #4', [o2, o4], db);
-
-  var o1 = {y_: 1, sourceBlock_: {}};
-  db.addConnection_(o1);
-  verify_DB_('Adding connection #1', [o1, o2, o4], db);
-
-  var o3a = {y_: 3, sourceBlock_: {}};
-  db.addConnection_(o3a);
-  verify_DB_('Adding connection #3a', [o1, o2, o3a, o4], db);
-
-  var o3b = {y_: 3, sourceBlock_: {}};
-  db.addConnection_(o3b);
-  verify_DB_('Adding connection #3b', [o1, o2, o3b, o3a, o4], db);
+function connectionTest_tearDown() {
+  input = null;
+  output = null;
+  previous = null;
+  next = null;
+  dummyWorkspace = null;
 }
 
-function test_DB_removeConnection() {
-  var db = new Blockly.ConnectionDB();
-  var o1 = {y_: 1, sourceBlock_: {}};
-  var o2 = {y_: 2, sourceBlock_: {}};
-  var o3a = {y_: 3, sourceBlock_: {}};
-  var o3b = {y_: 3, sourceBlock_: {}};
-  var o3c = {y_: 3, sourceBlock_: {}};
-  var o4 = {y_: 4, sourceBlock_: {}};
-  db.addConnection_(o1);
-  db.addConnection_(o2);
-  db.addConnection_(o3c);
-  db.addConnection_(o3b);
-  db.addConnection_(o3a);
-  db.addConnection_(o4);
-  verify_DB_('Adding connections 1-4', [o1, o2, o3a, o3b, o3c, o4], db);
+/**
+ * These tests check that the reasons for failures to connect are consistent (internal view of
+ * error states).
+ */
+function testCanConnectWithReason_TargetNull() {
+  connectionTest_setUp();
 
-  db.removeConnection_(o2);
-  verify_DB_('Removing connection #2', [o1, o3a, o3b, o3c, o4], db);
+  assertEquals(Blockly.Connection.REASON_TARGET_NULL, input.canConnectWithReason_(null));
 
-  db.removeConnection_(o4);
-  verify_DB_('Removing connection #4', [o1, o3a, o3b, o3c], db);
-
-  db.removeConnection_(o1);
-  verify_DB_('Removing connection #1', [o3a, o3b, o3c], db);
-
-  db.removeConnection_(o3a);
-  verify_DB_('Removing connection #3a', [o3b, o3c], db);
-
-  db.removeConnection_(o3c);
-  verify_DB_('Removing connection #3c', [o3b], db);
-
-  db.removeConnection_(o3b);
-  verify_DB_('Removing connection #3b', [], db);
+  connectionTest_tearDown();
 }
+
+function testCanConnectWithReason_Disconnect() {
+  connectionTest_setUp();
+
+  var tempConnection = new Blockly.Connection({workspace: dummyWorkspace}, Blockly.OUTPUT_VALUE);
+  Blockly.Connection.connectReciprocally(input, tempConnection);
+  assertEquals(Blockly.Connection.REASON_MUST_DISCONNECT, input.canConnectWithReason_(output));
+
+  connectionTest_tearDown();
+}
+
+function testCanConnectWithReason_DifferentWorkspaces() {
+  connectionTest_setUp();
+
+  input = new Blockly.Connection({workspace: {}}, Blockly.INPUT_VALUE);
+  output = new Blockly.Connection({workspace: dummyWorkspace}, Blockly.OUTPUT_VALUE);
+
+  assertEquals(Blockly.Connection.REASON_DIFFERENT_WORKSPACES, input.canConnectWithReason_(output));
+
+  connectionTest_tearDown();
+}
+
+
+function testCanConnectWithReason_Self() {
+  connectionTest_setUp();
+
+  var block = {type_: "test block"};
+  input.sourceBlock_ = block;
+  assertEquals(Blockly.Connection.REASON_SELF_CONNECTION, input.canConnectWithReason_(input));
+
+  connectionTest_tearDown();
+}
+
+function testCanConnectWithReason_Type() {
+  connectionTest_setUp();
+
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, input.canConnectWithReason_(previous));
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, input.canConnectWithReason_(next));
+
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, output.canConnectWithReason_(previous));
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, output.canConnectWithReason_(next));
+
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, previous.canConnectWithReason_(input));
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, previous.canConnectWithReason_(output));
+
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, next.canConnectWithReason_(input));
+  assertEquals(Blockly.Connection.REASON_WRONG_TYPE, next.canConnectWithReason_(output));
+
+  connectionTest_tearDown();
+}
+
+function testCanConnectWithReason_CanConnect() {
+  connectionTest_setUp();
+
+  assertEquals(Blockly.Connection.CAN_CONNECT, previous.canConnectWithReason_(next));
+  assertEquals(Blockly.Connection.CAN_CONNECT, next.canConnectWithReason_(previous));
+  assertEquals(Blockly.Connection.CAN_CONNECT, input.canConnectWithReason_(output));
+  assertEquals(Blockly.Connection.CAN_CONNECT, output.canConnectWithReason_(input));
+
+  connectionTest_tearDown();
+}
+
+/**
+ * The next set of tests checks that exceptions are being thrown at the correct times (external
+ * view of errors).
+ */
+function testCheckConnection_Self() {
+  connectionTest_setUp();
+  var block = {type_: "test block"};
+  input.sourceBlock_ = block;
+  try {
+    input.checkConnection_(input);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypeInputPrev() {
+  connectionTest_setUp();
+  try {
+    input.checkConnection_(previous);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypeInputNext() {
+  connectionTest_setUp();
+  try {
+    input.checkConnection_(next);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypeOutputPrev() {
+  connectionTest_setUp();
+  try {
+    output.checkConnection_(previous);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypePrevInput() {
+  connectionTest_setUp();
+  try {
+    previous.checkConnection_(input);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypePrevOutput() {
+  connectionTest_setUp();
+  try {
+    previous.checkConnection_(output);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypeNextInput() {
+  connectionTest_setUp();
+  try {
+    next.checkConnection_(input);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_TypeNextOutput() {
+  connectionTest_setUp();
+  try {
+    next.checkConnection_(output);
+    fail();
+  } catch (e) {
+    // expected
+  }
+
+  connectionTest_tearDown();
+}
+
+function testCheckConnection_Okay() {
+  connectionTest_setUp();
+  previous.checkConnection_(next);
+  next.checkConnection_(previous);
+  input.checkConnection_(output);
+  output.checkConnection_(input);
+
+  connectionTest_tearDown();
+}
\ No newline at end of file
diff --git a/tests/jsunit/index.html b/tests/jsunit/index.html
index 9c3df0ea6..48f134e89 100644
--- a/tests/jsunit/index.html
+++ b/tests/jsunit/index.html
@@ -10,6 +10,7 @@
     <script src="blockly_test.js"></script>
     <script src="block_test.js"></script>
     <script src="connection_test.js"></script>
+    <script src="connection_db_test.js"></script>
     <script src="generator_test.js"></script>
     <script src="names_test.js"></script>
     <script src="workspace_test.js"></script>