diff --git a/src/PubSub/FloodRouter.cs b/src/PubSub/FloodRouter.cs
index d111002..2ff32b1 100644
--- a/src/PubSub/FloodRouter.cs
+++ b/src/PubSub/FloodRouter.cs
@@ -5,6 +5,7 @@
 using Semver;
 using System;
 using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -21,7 +22,7 @@ public class FloodRouter : IPeerProtocol, IMessageRouter
         static ILog log = LogManager.GetLogger(typeof(FloodRouter));
 
         MessageTracker tracker = new MessageTracker();
-        List<string> localTopics = new List<string>();
+        ConcurrentDictionary<string, string> localTopics = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         ///   The topics of interest of other peers.
@@ -138,7 +139,7 @@ public IEnumerable<Peer> InterestedPeers(string topic)
         /// <inheritdoc />
         public async Task JoinTopicAsync(string topic, CancellationToken cancel)
         {
-            localTopics.Add(topic);
+            localTopics.TryAdd(topic, topic);
             var msg = new PubSubMessage
             {
                 Subscriptions = new Subscription[]
@@ -164,7 +165,7 @@ public async Task JoinTopicAsync(string topic, CancellationToken cancel)
         /// <inheritdoc />
         public async Task LeaveTopicAsync(string topic, CancellationToken cancel)
         {
-            localTopics.Remove(topic);
+            localTopics.TryRemove(topic, out _);
             var msg = new PubSubMessage
             {
                 Subscriptions = new Subscription[]
@@ -260,7 +261,7 @@ async void Swarm_ConnectionEstablished(object sender, PeerConnection connection)
             {
                 var hello = new PubSubMessage
                 {
-                    Subscriptions = localTopics
+                    Subscriptions = localTopics.Values
                         .Select(topic => new Subscription
                         {
                             Subscribe = true,
diff --git a/src/PubSub/NotificationService.cs b/src/PubSub/NotificationService.cs
index 0454aa0..9d54380 100644
--- a/src/PubSub/NotificationService.cs
+++ b/src/PubSub/NotificationService.cs
@@ -3,6 +3,7 @@
 using Ipfs.CoreApi;
 using System;
 using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.IO;
 using System.Linq;
 using System.Net;
@@ -30,7 +31,7 @@ class TopicHandler
         }
         
         long nextSequenceNumber;
-        List<TopicHandler> topicHandlers;
+        ConcurrentDictionary<TopicHandler, TopicHandler> topicHandlers;
         MessageTracker tracker = new MessageTracker();
         
         // TODO: A general purpose CancellationTokenSource that stops publishing of
@@ -67,7 +68,7 @@ class TopicHandler
         /// <inheritdoc />
         public async Task StartAsync()
         {
-            topicHandlers = new List<TopicHandler>();
+            topicHandlers = new ConcurrentDictionary<TopicHandler, TopicHandler>();
 
             // Resolution of 100 nanoseconds.
             nextSequenceNumber = DateTime.UtcNow.Ticks;
@@ -133,7 +134,7 @@ public PublishedMessage CreateMessage(string topic, byte[] data)
         /// <inheritdoc />
         public Task<IEnumerable<string>> SubscribedTopicsAsync(CancellationToken cancel = default(CancellationToken))
         {
-            var topics = topicHandlers
+            var topics = topicHandlers.Values
                 .Select(t => t.Topic)
                 .Distinct();
             return Task.FromResult(topics);
@@ -179,14 +180,14 @@ public PublishedMessage CreateMessage(string topic, byte[] data)
         public async Task SubscribeAsync(string topic, Action<IPublishedMessage> handler, CancellationToken cancellationToken)
         {
             var topicHandler = new TopicHandler { Topic = topic, Handler = handler };
-            topicHandlers.Add(topicHandler);
+            topicHandlers.TryAdd(topicHandler, topicHandler);
 
             // TODO: need a better way.
 #pragma warning disable VSTHRD101 
             cancellationToken.Register(async () =>
             {
-                topicHandlers.Remove(topicHandler);
-                if (topicHandlers.Count(t => t.Topic == topic) == 0)
+                topicHandlers.TryRemove(topicHandler, out _);
+                if (topicHandlers.Values.Count(t => t.Topic == topic) == 0)
                 {
                     await Task.WhenAll(Routers.Select(r => r.LeaveTopicAsync(topic, CancellationToken.None))).ConfigureAwait(false);
                 }
@@ -194,7 +195,7 @@ public async Task SubscribeAsync(string topic, Action<IPublishedMessage> handler
 #pragma warning restore VSTHRD101 
 
             // Tell routers if first time.
-            if (topicHandlers.Count(t => t.Topic == topic) == 1)
+            if (topicHandlers.Values.Count(t => t.Topic == topic) == 1)
             {
                 await Task.WhenAll(Routers.Select(r => r.JoinTopicAsync(topic, CancellationToken.None))).ConfigureAwait(false);
             }
@@ -224,7 +225,7 @@ void Router_MessageReceived(object sender, PublishedMessage msg)
             }
 
             // Call local topic handlers.
-            var handlers = topicHandlers
+            var handlers = topicHandlers.Values
                 .Where(th => msg.Topics.Contains(th.Topic));
             foreach (var handler in handlers)
             {