diff --git a/objectivec/GPBExtensionRegistry.m b/objectivec/GPBExtensionRegistry.m
new file mode 100644
index 0000000..df61a17
--- /dev/null
+++ b/objectivec/GPBExtensionRegistry.m
@@ -0,0 +1,108 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#import "GPBExtensionRegistry.h"
+
+#import "GPBBootstrap.h"
+#import "GPBDescriptor.h"
+
+@implementation GPBExtensionRegistry {
+  // TODO(dmaclach): Reimplement with CFDictionaries that don't use
+  // objects as keys.
+  NSMutableDictionary *mutableClassMap_;
+}
+
+- (instancetype)init {
+  if ((self = [super init])) {
+    mutableClassMap_ = [[NSMutableDictionary alloc] init];
+  }
+  return self;
+}
+
+- (void)dealloc {
+  [mutableClassMap_ release];
+  [super dealloc];
+}
+
+- (instancetype)copyWithZone:(NSZone *)zone {
+  GPBExtensionRegistry *result = [[[self class] allocWithZone:zone] init];
+  if (result && mutableClassMap_.count) {
+    [result->mutableClassMap_ addEntriesFromDictionary:mutableClassMap_];
+  }
+  return result;
+}
+
+- (NSMutableDictionary *)extensionMapForContainingMessageClass:
+        (Class)containingMessageClass {
+  NSMutableDictionary *extensionMap =
+      [mutableClassMap_ objectForKey:containingMessageClass];
+  if (extensionMap == nil) {
+    extensionMap = [NSMutableDictionary dictionary];
+    [mutableClassMap_ setObject:extensionMap
+                         forKey:(id<NSCopying>)containingMessageClass];
+  }
+  return extensionMap;
+}
+
+- (void)addExtension:(GPBExtensionDescriptor *)extension {
+  if (extension == nil) {
+    return;
+  }
+
+  Class containingMessageClass = extension.containingMessageClass;
+  NSMutableDictionary *extensionMap =
+      [self extensionMapForContainingMessageClass:containingMessageClass];
+  [extensionMap setObject:extension forKey:@(extension.fieldNumber)];
+}
+
+- (GPBExtensionDescriptor *)extensionForDescriptor:(GPBDescriptor *)descriptor
+                                       fieldNumber:(NSInteger)fieldNumber {
+  Class messageClass = descriptor.messageClass;
+  NSDictionary *extensionMap =
+      [mutableClassMap_ objectForKey:messageClass];
+  return [extensionMap objectForKey:@(fieldNumber)];
+}
+
+- (void)addExtensions:(GPBExtensionRegistry *)registry {
+  if (registry == nil) {
+    // In the case where there are no extensions just ignore.
+    return;
+  }
+  NSMutableDictionary *otherClassMap = registry->mutableClassMap_;
+  for (Class containingMessageClass in otherClassMap) {
+    NSMutableDictionary *extensionMap =
+        [self extensionMapForContainingMessageClass:containingMessageClass];
+    NSMutableDictionary *otherExtensionMap =
+        [registry extensionMapForContainingMessageClass:containingMessageClass];
+    [extensionMap addEntriesFromDictionary:otherExtensionMap];
+  }
+}
+
+@end