diff --git a/core-api/src/main/java/com/optimizely/ab/notification/NotificationManager.java b/core-api/src/main/java/com/optimizely/ab/notification/NotificationManager.java
index 5254d76b8..7415e6b23 100644
--- a/core-api/src/main/java/com/optimizely/ab/notification/NotificationManager.java
+++ b/core-api/src/main/java/com/optimizely/ab/notification/NotificationManager.java
@@ -19,6 +19,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -33,7 +34,7 @@ public class NotificationManager<T> {
 
     private static final Logger logger = LoggerFactory.getLogger(NotificationManager.class);
 
-    private final Map<Integer, NotificationHandler<T>> handlers = new LinkedHashMap<>();
+    private final Map<Integer, NotificationHandler<T>> handlers = Collections.synchronizedMap(new LinkedHashMap<>());
     private final AtomicInteger counter;
 
     public NotificationManager() {
@@ -47,10 +48,12 @@ public NotificationManager(AtomicInteger counter) {
     public int addHandler(NotificationHandler<T> newHandler) {
 
         // Prevent registering a duplicate listener.
-        for (NotificationHandler<T> handler: handlers.values()) {
-            if (handler.equals(newHandler)) {
-                logger.warn("Notification listener was already added");
-                return -1;
+        synchronized (handlers) {
+            for (NotificationHandler<T> handler : handlers.values()) {
+                if (handler.equals(newHandler)) {
+                    logger.warn("Notification listener was already added");
+                    return -1;
+                }
             }
         }
 
@@ -61,11 +64,13 @@ public int addHandler(NotificationHandler<T> newHandler) {
     }
 
     public void send(final T message) {
-        for (Map.Entry<Integer, NotificationHandler<T>> handler: handlers.entrySet()) {
-            try {
-                handler.getValue().handle(message);
-            } catch (Exception e) {
-                logger.warn("Catching exception sending notification for class: {}, handler: {}", message.getClass(), handler.getKey());
+        synchronized (handlers) {
+            for (Map.Entry<Integer, NotificationHandler<T>> handler: handlers.entrySet()) {
+                try {
+                    handler.getValue().handle(message);
+                } catch (Exception e) {
+                    logger.warn("Catching exception sending notification for class: {}, handler: {}", message.getClass(), handler.getKey());
+                }
             }
         }
     }
diff --git a/core-api/src/test/java/com/optimizely/ab/notification/NotificationManagerTest.java b/core-api/src/test/java/com/optimizely/ab/notification/NotificationManagerTest.java
index c51a84e3f..58767ac7a 100644
--- a/core-api/src/test/java/com/optimizely/ab/notification/NotificationManagerTest.java
+++ b/core-api/src/test/java/com/optimizely/ab/notification/NotificationManagerTest.java
@@ -20,6 +20,11 @@
 import org.junit.Test;
 
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import static org.junit.Assert.*;
@@ -70,4 +75,32 @@ public void testSendWithError() {
         assertEquals(1, messages.size());
         assertEquals("message1", messages.get(0).getMessage());
     }
+
+    @Test
+    public void testThreadSafety() throws InterruptedException {
+        int numThreads = 10;
+        int numRepeats = 2;
+        ExecutorService executor = Executors.newFixedThreadPool(numThreads);
+        CountDownLatch latch = new CountDownLatch(numThreads);
+        AtomicBoolean failedAlready = new AtomicBoolean(false);
+
+        for(int i = 0; i < numThreads; i++) {
+            executor.execute(() -> {
+                try {
+                    for (int j = 0; j < numRepeats; j++) {
+                        if(!failedAlready.get()) {
+                            notificationManager.addHandler(new TestNotificationHandler<>());
+                            notificationManager.send(new TestNotification("message1"));
+                        }
+                    }
+                } catch (Exception e) {
+                    failedAlready.set(true);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+        assertTrue(latch.await(10, TimeUnit.SECONDS));
+        assertEquals(numThreads * numRepeats, notificationManager.size());
+    }
 }