diff --git a/lib/lita-slack.rb b/lib/lita-slack.rb
index 85426dd..abafffc 100644
--- a/lib/lita-slack.rb
+++ b/lib/lita-slack.rb
@@ -4,4 +4,5 @@
   File.join("..", "..", "locales", "*.yml"), __FILE__
 )]
 
-require "lita/adapters/slack"
+require_relative "lita/source"
+require_relative "lita/adapters/slack"
diff --git a/lib/lita/adapters/slack.rb b/lib/lita/adapters/slack.rb
index 6107cb6..f98eb36 100644
--- a/lib/lita/adapters/slack.rb
+++ b/lib/lita/adapters/slack.rb
@@ -1,5 +1,5 @@
-require 'lita/adapters/slack/chat_service'
-require 'lita/adapters/slack/rtm_connection'
+require_relative 'slack/chat_service'
+require_relative 'slack/rtm_connection'
 
 module Lita
   module Adapters
@@ -40,7 +40,11 @@ def roster(target)
 
       def send_messages(target, strings)
         api = API.new(config)
-        api.send_messages(channel_for(target), strings)
+        if target.respond_to?(:thread)
+          api.send_messages(target.room || target.user&.id, strings, thread_ts: target.thread)
+        else
+          api.send_messages(target.room || target.user&.id, strings)
+        end
       end
 
       def set_topic(target, topic)
@@ -60,14 +64,6 @@ def shut_down
 
       attr_reader :rtm_connection
 
-      def channel_for(target)
-        if target.private_message?
-          rtm_connection.im_for(target.user.id)
-        else
-          target.room
-        end
-      end
-
       def channel_roster(room_id, api)
         response = api.channels_info room_id
         response['channel']['members']
diff --git a/lib/lita/adapters/slack/api.rb b/lib/lita/adapters/slack/api.rb
index 3bb437c..8567a46 100644
--- a/lib/lita/adapters/slack/api.rb
+++ b/lib/lita/adapters/slack/api.rb
@@ -1,9 +1,9 @@
 require 'faraday'
 
-require 'lita/adapters/slack/team_data'
-require 'lita/adapters/slack/slack_im'
-require 'lita/adapters/slack/slack_user'
-require 'lita/adapters/slack/slack_channel'
+require_relative 'team_data'
+require_relative 'slack_im'
+require_relative 'slack_user'
+require_relative 'slack_channel'
 
 module Lita
   module Adapters
@@ -55,29 +55,31 @@ def send_attachments(room_or_user, attachments)
           )
         end
 
-        def send_messages(channel_id, messages)
-          call_api(
-            "chat.postMessage",
-            **post_message_config,
-            as_user: true,
+        def send_messages(channel_id, messages, additional_payload = {})
+          data = {
             channel: channel_id,
+            as_user: true,
             text: messages.join("\n"),
-          )
+          }.merge(post_message_config)
+           .merge(additional_payload)
+
+          call_api( "chat.postMessage", **data )
         end
 
         def set_topic(channel, topic)
           call_api("channels.setTopic", channel: channel, topic: topic)
         end
 
-        def rtm_start
-          response_data = call_api("rtm.start")
+        def rtm_connect
+          response_data = call_api("rtm.connect")
+
+          raise RuntimeError, response_data["error"] if response_data["ok"] != true
 
           TeamData.new(
-            SlackIM.from_data_array(response_data["ims"]),
+            response_data["team"]["id"],
+            response_data["team"]["name"],
+            response_data["team"]["domain"],
             SlackUser.from_data(response_data["self"]),
-            SlackUser.from_data_array(response_data["users"]),
-            SlackChannel.from_data_array(response_data["channels"]) +
-              SlackChannel.from_data_array(response_data["groups"]),
             response_data["url"],
           )
         end
diff --git a/lib/lita/adapters/slack/chat_service.rb b/lib/lita/adapters/slack/chat_service.rb
index 317997b..48b7475 100644
--- a/lib/lita/adapters/slack/chat_service.rb
+++ b/lib/lita/adapters/slack/chat_service.rb
@@ -1,4 +1,4 @@
-require "lita/adapters/slack/attachment"
+require_relative "attachment"
 
 module Lita
   module Adapters
diff --git a/lib/lita/adapters/slack/message_handler.rb b/lib/lita/adapters/slack/message_handler.rb
index b1572b2..721db71 100644
--- a/lib/lita/adapters/slack/message_handler.rb
+++ b/lib/lita/adapters/slack/message_handler.rb
@@ -112,9 +112,13 @@ def channel
           data["channel"]
         end
 
+        def thread
+          data["thread_ts"]
+        end
+
         def dispatch_message(user)
           room = Lita::Room.find_by_id(channel)
-          source = Source.new(user: user, room: room || channel)
+          source = Source.new(user: user, room: room || channel, thread: thread)
           source.private_message! if channel && channel[0] == "D"
           message = Message.new(robot, body, source)
           message.command! if source.private_message?
diff --git a/lib/lita/adapters/slack/rtm_connection.rb b/lib/lita/adapters/slack/rtm_connection.rb
index 90bae4d..cb9f661 100644
--- a/lib/lita/adapters/slack/rtm_connection.rb
+++ b/lib/lita/adapters/slack/rtm_connection.rb
@@ -1,12 +1,12 @@
 require 'faye/websocket'
 require 'multi_json'
 
-require 'lita/adapters/slack/api'
-require 'lita/adapters/slack/event_loop'
-require 'lita/adapters/slack/im_mapping'
-require 'lita/adapters/slack/message_handler'
-require 'lita/adapters/slack/room_creator'
-require 'lita/adapters/slack/user_creator'
+require_relative 'api'
+require_relative 'event_loop'
+require_relative 'im_mapping'
+require_relative 'message_handler'
+require_relative 'room_creator'
+require_relative 'user_creator'
 
 module Lita
   module Adapters
@@ -17,23 +17,15 @@ class RTMConnection
 
         class << self
           def build(robot, config)
-            new(robot, config, API.new(config).rtm_start)
+            new(robot, config, API.new(config).rtm_connect)
           end
         end
 
         def initialize(robot, config, team_data)
           @robot = robot
           @config = config
-          @im_mapping = IMMapping.new(API.new(config), team_data.ims)
           @websocket_url = team_data.websocket_url
           @robot_id = team_data.self.id
-
-          UserCreator.create_users(team_data.users, robot, robot_id)
-          RoomCreator.create_rooms(team_data.channels, robot)
-        end
-
-        def im_for(user_id)
-          im_mapping.im_for(user_id)
         end
 
         def run(queue = nil, options = {})
diff --git a/lib/lita/adapters/slack/team_data.rb b/lib/lita/adapters/slack/team_data.rb
index e77b3b3..8cdb326 100644
--- a/lib/lita/adapters/slack/team_data.rb
+++ b/lib/lita/adapters/slack/team_data.rb
@@ -2,7 +2,7 @@ module Lita
   module Adapters
     # @api private
     class Slack < Adapter
-      TeamData = Struct.new(:ims, :self, :users, :channels, :websocket_url)
+      TeamData = Struct.new(:id, :name, :domain, :self, :websocket_url)
     end
   end
 end
diff --git a/lib/lita/source.rb b/lib/lita/source.rb
new file mode 100644
index 0000000..72c826e
--- /dev/null
+++ b/lib/lita/source.rb
@@ -0,0 +1,35 @@
+module Lita
+  class Source
+
+    # The room the message came from or should be sent to, as a string.
+    # @return [String, NilClass] A string uniquely identifying the room.
+    attr_reader :thread
+
+    # @param user [Lita::User] The user who sent the message or should receive
+    #   the outgoing message.
+    # @param room [Lita::Room, String] A string or {Lita::Room} uniquely identifying the room
+    #   the user sent the message from, or the room where a reply should go. The format of this
+    #   string (or the ID of the {Lita::Room} object) will differ depending on the chat service.
+    # @param private_message [Boolean] A flag indicating whether or not the
+    #   message was sent privately.
+    def initialize(user: nil, room: nil, thread: nil, private_message: false)
+      @user = user
+      @thread = thread
+
+      case room
+      when String
+        @room = room
+        @room_object = Room.new(room)
+      when Room
+        @room = room.id
+        @room_object = room
+      end
+
+      @private_message = private_message
+
+      raise ArgumentError, I18n.t("lita.source.user_or_room_required") if user.nil? && room.nil?
+
+      @private_message = true if room.nil?
+    end
+  end
+end
diff --git a/lita-slack.gemspec b/lita-slack.gemspec
index 59a929f..0c0aa55 100644
--- a/lita-slack.gemspec
+++ b/lita-slack.gemspec
@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "lita", ">= 4.7.1"
   spec.add_runtime_dependency "multi_json"
 
-  spec.add_development_dependency "bundler", "~> 1.3"
+  spec.add_development_dependency "bundler"
   spec.add_development_dependency "pry-byebug"
   spec.add_development_dependency "rack-test"
   spec.add_development_dependency "rake"
diff --git a/spec/lita/adapters/slack/api_spec.rb b/spec/lita/adapters/slack/api_spec.rb
index 771df2e..a012369 100644
--- a/spec/lita/adapters/slack/api_spec.rb
+++ b/spec/lita/adapters/slack/api_spec.rb
@@ -637,11 +637,11 @@ def stubs(postMessage_options = {})
     end
   end
 
-  describe "#rtm_start" do
+  describe "#rtm_connect" do
     let(:http_status) { 200 }
     let(:stubs) do
       Faraday::Adapter::Test::Stubs.new do |stub|
-        stub.post('https://slack.com/api/rtm.start', token: token) do
+        stub.post('https://slack.com/api/rtm.connect', token: token) do
           [http_status, {}, http_response]
         end
       end
@@ -652,34 +652,19 @@ def stubs(postMessage_options = {})
         MultiJson.dump({
           ok: true,
           url: 'wss://example.com/',
-          users: [{ id: 'U023BECGF' }],
-          ims: [{ id: 'D024BFF1M' }],
           self: { id: 'U12345678' },
-          channels: [{ id: 'C1234567890' }],
-          groups: [{ id: 'G0987654321' }],
+          team: { id: 'T0987654321' },
         })
       end
 
       it "has data on the bot user" do
-        response = subject.rtm_start
+        response = subject.rtm_connect
 
         expect(response.self.id).to eq('U12345678')
       end
 
-      it "has an array of IMs" do
-        response = subject.rtm_start
-
-        expect(response.ims[0].id).to eq('D024BFF1M')
-      end
-
-      it "has an array of users" do
-        response = subject.rtm_start
-
-        expect(response.users[0].id).to eq('U023BECGF')
-      end
-
       it "has a WebSocket URL" do
-        response = subject.rtm_start
+        response = subject.rtm_connect
 
         expect(response.websocket_url).to eq('wss://example.com/')
       end
diff --git a/spec/lita/adapters/slack/message_handler_spec.rb b/spec/lita/adapters/slack/message_handler_spec.rb
index c449b0c..aae8568 100644
--- a/spec/lita/adapters/slack/message_handler_spec.rb
+++ b/spec/lita/adapters/slack/message_handler_spec.rb
@@ -1,10 +1,13 @@
 require "spec_helper"
+# require_relative '../../../../lib/lita/adapters/slack/slack_source'
 
 describe Lita::Adapters::Slack::MessageHandler, lita: true do
   subject { described_class.new(robot, robot_id, data) }
 
   before do
     allow(robot).to receive(:trigger)
+    allow(robot).to receive(:alias)
+    allow(robot).to receive(:receive)
     Lita::Adapters::Slack::RoomCreator.create_room(channel, robot)
   end
 
@@ -44,7 +47,8 @@
         allow(Lita::Room).to receive(:find_by_id).and_return(room)
         allow(Lita::Source).to receive(:new).with(
           user: user,
-          room: room
+          room: room,
+          thread: nil,
         ).and_return(source)
         allow(Lita::Message).to receive(:new).with(robot, "Hello", source).and_return(message)
         allow(robot).to receive(:receive).with(message)
@@ -74,7 +78,8 @@
           allow(Lita::Room).to receive(:find_by_id).and_return(nil)
           allow(Lita::Source).to receive(:new).with(
             user: user,
-            room: "D2147483705"
+            room: "D2147483705",
+            thread: nil,
           ).and_return(source)
           allow(source).to receive(:private_message!).and_return(true)
           allow(source).to receive(:private_message?).and_return(true)
@@ -156,6 +161,32 @@
         end
       end
 
+      context "when the message is in a thread" do
+        let(:data) do
+          {
+            "type" => "message",
+            "channel" => "C2147483705",
+            "user" => "U023BECGF",
+            "text" => "Hello",
+            "ts" => "1234.5678",
+            "thread_ts" => "1234.5678",
+          }
+        end
+        let(:source) { instance_double('Lita::Source', room: room, private_message?: false) }
+
+        before do
+          allow(Lita::Source).to receive(:new).with(
+            user: user,
+            room: room,
+            thread: "1234.5678",
+          ).and_return(source)
+        end
+
+        it "records the room thread" do
+          subject.handle
+        end
+      end
+
       describe "Removing message formatting" do
 
         let(:user) { instance_double('Lita::User', id: 'U123',name: 'name', mention_name: 'label') }
diff --git a/spec/lita/adapters/slack/rtm_connection_spec.rb b/spec/lita/adapters/slack/rtm_connection_spec.rb
index 32188f3..6219fd7 100644
--- a/spec/lita/adapters/slack/rtm_connection_spec.rb
+++ b/spec/lita/adapters/slack/rtm_connection_spec.rb
@@ -9,21 +9,18 @@ def with_websocket(subject, queue)
     thread.join
   end
 
-  subject { described_class.new(robot, config, rtm_start_response) }
+  subject { described_class.new(robot, config, rtm_connect_response) }
 
   let(:api) { instance_double("Lita::Adapters::Slack::API") }
   let(:registry) { Lita::Registry.new }
   let(:robot) { Lita::Robot.new(registry) }
-  let(:raw_user_data) { Hash.new }
-  let(:channel) { Lita::Adapters::Slack::SlackChannel.new('C2147483705', 'general', 1360782804, 'U023BECGF', metadata) }
-  let(:metadata) { Hash.new }
 
-  let(:rtm_start_response) do
+  let(:rtm_connect_response) do
     Lita::Adapters::Slack::TeamData.new(
-      [],
-      Lita::Adapters::Slack::SlackUser.new('U12345678', 'carl', nil, raw_user_data),
-      [Lita::Adapters::Slack::SlackUser.new('U12345678', 'carl', '', raw_user_data)],
-      [channel],
+      'T2U81E2FP',
+      'SlackDemo',
+      'slackdemo',
+      Lita::Adapters::Slack::SlackUser.new('U12345678', 'carl', nil, {}),
       "wss://example.com/"
     )
   end
@@ -39,42 +36,12 @@ def with_websocket(subject, queue)
   describe ".build" do
     before do
       allow(Lita::Adapters::Slack::API).to receive(:new).with(config).and_return(api)
-      allow(api).to receive(:rtm_start).and_return(rtm_start_response)
+      allow(api).to receive(:rtm_connect).and_return(rtm_connect_response)
     end
 
-    it "constructs a new RTMConnection with the results of rtm.start data" do
+    it "constructs a new RTMConnection with the results of rtm.connect data" do
       expect(described_class.build(robot, config)).to be_an_instance_of(described_class)
     end
-
-    it "creates users with the results of rtm.start data" do
-      expect(Lita::Adapters::Slack::UserCreator).to receive(:create_users)
-
-      described_class.build(robot, config)
-    end
-
-    it "creates rooms with the results of rtm.start data" do
-      expect(Lita::Adapters::Slack::RoomCreator).to receive(:create_rooms)
-
-      described_class.build(robot, config)
-    end
-  end
-
-  describe "#im_for" do
-    before do
-      allow(Lita::Adapters::Slack::API).to receive(:new).with(config).and_return(api)
-      allow(
-        Lita::Adapters::Slack::IMMapping
-      ).to receive(:new).with(api, []).and_return(im_mapping)
-      allow(im_mapping).to receive(:im_for).with('U12345678').and_return('D024BFF1M')
-    end
-
-    let(:im_mapping) { instance_double('Lita::Adapters::Slack::IMMapping') }
-
-    it "delegates to the IMMapping" do
-      with_websocket(subject, queue) do |websocket|
-        expect(subject.im_for('U12345678')).to eq('D024BFF1M')
-      end
-    end
   end
 
   describe "#run" do
@@ -118,6 +85,7 @@ def with_websocket(subject, queue)
     context "when the WebSocket is closed from outside" do
       it "shuts down the reactor" do
         with_websocket(subject, queue) do |websocket|
+            sleep 0.1 # Since this code is run in a thread, we need to wait for it to finish
             websocket.close
             expect(EM.stopping?).to be_truthy
           end
diff --git a/spec/lita/adapters/slack_spec.rb b/spec/lita/adapters/slack_spec.rb
index fbbbd23..830063b 100644
--- a/spec/lita/adapters/slack_spec.rb
+++ b/spec/lita/adapters/slack_spec.rb
@@ -133,10 +133,37 @@
 
       it "does not send via the RTM api" do
         expect(rtm_connection).to_not receive(:send_messages)
-        expect(api).to receive(:send_messages).with(room_source.room, ['foo'])
+        expect(api).to receive(:send_messages).with(room_source.room, ['foo'], { thread_ts: nil })
 
         subject.send_messages(room_source, ['foo'])
       end
+
+      context "when thread is set" do
+        let(:room) { Lita::Room.new('C024BE91L') }
+        let(:room_source) { Lita::Source.new(room: room, thread: '12345.67890') }
+
+        it "sends the message to the Web API with thread_ts" do
+          expect(api).to receive(:send_messages).with(room_source.room, ['foo'], { thread_ts: '12345.67890' })
+
+          subject.send_messages(room_source, ['foo'])
+        end
+      end
+
+      context "with optional thread" do
+        it "sends the message to the Web API without thread_ts" do
+          expect(api).to receive(:send_messages).with(private_message_source.room, ['foo'])
+
+          subject.send_messages(private_message_source, ['foo'])
+        end
+      end
+
+      context "with user source" do
+        it "sends direct message to the Web API" do
+          expect(api).to receive(:send_messages).with(user_source.user.id, ['foo'])
+
+          subject.send_messages(user_source, ['foo'])
+        end
+      end
     end
   end
 
diff --git a/spec/lita/adapters/source_spec.rb b/spec/lita/adapters/source_spec.rb
new file mode 100644
index 0000000..170967e
--- /dev/null
+++ b/spec/lita/adapters/source_spec.rb
@@ -0,0 +1,10 @@
+require "spec_helper"
+
+describe Lita::Source do
+  subject { described_class.new(room: 'R0000', thread: 'thread') }
+
+  it 'records message source thread' do
+    expect(subject.thread).to eq('thread')
+  end
+
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 6632cfb..d52bf12 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -6,7 +6,7 @@
 ]
 SimpleCov.start { add_filter "/spec/" }
 
-require "lita-slack"
+require_relative "../lib/lita-slack"
 require "lita/rspec"
 
 require "pry"