diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/Logging.kt b/backend/src/main/java/net/buzzert/kordophone/backend/Logging.kt
deleted file mode 100644
index 4c88f34..0000000
--- a/backend/src/main/java/net/buzzert/kordophone/backend/Logging.kt
+++ /dev/null
@@ -1,2 +0,0 @@
-package net.buzzert.kordophone.backend
-
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt b/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt
new file mode 100644
index 0000000..eff9dd2
--- /dev/null
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt
@@ -0,0 +1,9 @@
+package net.buzzert.kordophone.backend.events
+
+import net.buzzert.kordophone.backend.model.Conversation
+import net.buzzert.kordophone.backend.model.Message
+
+data class MessageDeliveredEvent(
+    val message: Message,
+    val conversation: Conversation,
+)
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/server/APIInterface.kt b/backend/src/main/java/net/buzzert/kordophone/backend/server/APIInterface.kt
index d17a5ff..71a951a 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/server/APIInterface.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/server/APIInterface.kt
@@ -1,13 +1,27 @@
 package net.buzzert.kordophone.backend.server
 
+import com.google.gson.annotations.SerializedName
 import net.buzzert.kordophone.backend.model.Conversation
 import net.buzzert.kordophone.backend.model.Message
 import okhttp3.ResponseBody
 import retrofit2.Call
 import retrofit2.Response
+import retrofit2.http.Body
 import retrofit2.http.GET
+import retrofit2.http.POST
 import retrofit2.http.Query
 
+data class SendMessageRequest(
+    @SerializedName("guid")
+    val conversationGUID: String,
+
+    @SerializedName("body")
+    val body: String,
+
+    @SerializedName("fileTransferGUIDs")
+    val transferGUIDs: List<String>?,
+)
+
 interface APIInterface {
     @GET("/version")
     suspend fun getVersion(): ResponseBody
@@ -17,4 +31,7 @@ interface APIInterface {
 
     @GET("/messages")
     suspend fun getMessages(@Query("guid") conversationGUID: String): Response<List<Message>>
+
+    @POST("/sendMessage")
+    suspend fun sendMessage(@Body request: SendMessageRequest): Response<Void>
 }
\ No newline at end of file
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt b/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt
index 19593d6..ec549b1 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt
@@ -1,14 +1,22 @@
 package net.buzzert.kordophone.backend.server
 
+import android.util.Log
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.runBlocking
+import net.buzzert.kordophone.backend.events.MessageDeliveredEvent
 import net.buzzert.kordophone.backend.model.Conversation
 import net.buzzert.kordophone.backend.model.Message
 import java.net.URL
+import java.util.Queue
+import java.util.concurrent.ArrayBlockingQueue
 
-class ChatRepository(apiInterface: APIInterface) {
-    // private val client: APIClient = APIClient(baseURL)
-    // private val apiInterface: APIInterface = client.getClient().create(APIInterface::class.java)
+const val REPO_LOG: String = "ChatRepository"
 
-    private val apiInterface: APIInterface = apiInterface
+class ChatRepository(private val apiInterface: APIInterface) {
+    public val messageDeliveredChannel = Channel<MessageDeliveredEvent>()
+
+    private val outgoingMessageQueue: ArrayBlockingQueue<Pair<Message, Conversation>> = ArrayBlockingQueue(16)
+    private var outgoingMessageThread: Thread? = null
 
     suspend fun getVersion(): String {
         return apiInterface.getVersion().string()
@@ -21,4 +29,44 @@ class ChatRepository(apiInterface: APIInterface) {
     suspend fun fetchMessages(conversation: Conversation): List<Message> {
         return apiInterface.getMessages(conversation.guid).body()!!
     }
+
+    suspend fun enqueueOutgoingMessage(message: Message, conversation: Conversation) {
+        Log.d(REPO_LOG, "Enqueuing outgoing message: $message")
+        outgoingMessageQueue.add(Pair(message, conversation))
+
+        if (outgoingMessageThread == null) {
+            outgoingMessageThread = Thread { outgoingMessageQueueMain() }
+            outgoingMessageThread?.start()
+        }
+    }
+
+    // - private
+
+    private fun outgoingMessageQueueMain() {
+        Log.d(REPO_LOG, "Outgoing Message Queue Main")
+        while (true) {
+            val outgoingMessageRequest = outgoingMessageQueue.poll()?.let {
+                runBlocking {
+                    val (message, conversation) = it
+                    Log.d(REPO_LOG, "Sending message to $conversation: $message")
+
+                    val result = apiInterface.sendMessage(
+                        SendMessageRequest(
+                            conversationGUID = conversation.guid,
+                            body = message.text,
+                            transferGUIDs = null,
+                        )
+                    )
+
+                    if (result.isSuccessful) {
+                        Log.d(REPO_LOG, "Successfully sent message.")
+                        messageDeliveredChannel.send(MessageDeliveredEvent(message, conversation))
+                    } else {
+                        Log.e(REPO_LOG, "Error sending message. Enqueuing for retry.")
+                        outgoingMessageQueue.add(it)
+                    }
+                }
+            }
+        }
+    }
 }
\ No newline at end of file
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/server/MockServer.kt b/backend/src/main/java/net/buzzert/kordophone/backend/server/MockServer.kt
index 76c8603..bb336b3 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/server/MockServer.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/server/MockServer.kt
@@ -5,6 +5,8 @@ import net.buzzert.kordophone.backend.model.Message
 import okhttp3.MediaType
 import okhttp3.ResponseBody
 import retrofit2.Response
+import java.util.Date
+import java.util.UUID
 
 class MockServer: APIInterface {
     val version = "Kordophone-2.0"
@@ -23,6 +25,19 @@ class MockServer: APIInterface {
         return Response.success(messages[conversationGUID])
     }
 
+    override suspend fun sendMessage(request: SendMessageRequest): Response<Void> {
+        messages[request.conversationGUID]!!.add(
+            Message(
+                text = request.body,
+                date = Date(),
+                guid = UUID.randomUUID().toString(),
+                sender = null, // me
+            )
+        )
+
+        return Response.success(null)
+    }
+
     fun addConversation(conversation: Conversation) {
         conversations.add(conversation)
     }
diff --git a/backend/src/test/java/net/buzzert/kordophone/backend/ExampleUnitTest.kt b/backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt
similarity index 78%
rename from backend/src/test/java/net/buzzert/kordophone/backend/ExampleUnitTest.kt
rename to backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt
index ff20f21..75992f3 100644
--- a/backend/src/test/java/net/buzzert/kordophone/backend/ExampleUnitTest.kt
+++ b/backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt
@@ -7,19 +7,13 @@ import net.buzzert.kordophone.backend.server.APIClient
 import net.buzzert.kordophone.backend.server.APIInterface
 import net.buzzert.kordophone.backend.server.ChatRepository
 import net.buzzert.kordophone.backend.server.MockServer
+import org.junit.Assert.assertEquals
 import org.junit.Test
-
-import org.junit.Assert.*
 import java.net.URL
 import java.util.Date
 import java.util.UUID
 
-/**
- * Example local unit test, which will execute on the development machine (host).
- *
- * See [testing documentation](http://d.android.com/tools/testing).
- */
-class ExampleUnitTest {
+class BackendTests {
     private val mockServer = MockServer().also {
         val conversation = Conversation(
             date = Date(),
@@ -74,4 +68,19 @@ class ExampleUnitTest {
         assertEquals(message.text, "Hey")
     }
 
+    @Test
+    fun testSendMessage() = runBlocking {
+        val conversation = repository.fetchConversations().first()
+        val outgoingMessage = Message(
+            date = Date(),
+            text = "Hello there!",
+            guid = UUID.randomUUID().toString(),
+            sender = null,
+        )
+
+        repository.enqueueOutgoingMessage(outgoingMessage, conversation)
+
+        val event = repository.messageDeliveredChannel.receive()
+        assertEquals(event.message.text, outgoingMessage.text)
+    }
 }
\ No newline at end of file
diff --git a/backend/src/test/java/net/buzzert/kordophone/backend/java/android/util/Log.kt b/backend/src/test/java/net/buzzert/kordophone/backend/java/android/util/Log.kt
new file mode 100644
index 0000000..10832a6
--- /dev/null
+++ b/backend/src/test/java/net/buzzert/kordophone/backend/java/android/util/Log.kt
@@ -0,0 +1,29 @@
+package android.util
+
+public class Log {
+    companion object {
+        @JvmStatic
+        fun d(tag: String, msg: String): Int {
+            println("DEBUG: $tag: $msg")
+            return 0
+        }
+
+        @JvmStatic
+        fun i(tag: String, msg: String): Int {
+            println("INFO: $tag: $msg")
+            return 0
+        }
+
+        @JvmStatic
+        fun w(tag: String, msg: String): Int {
+            println("WARN: $tag: $msg")
+            return 0
+        }
+
+        @JvmStatic
+        fun e(tag: String, msg: String): Int {
+            println("ERROR: $tag: $msg")
+            return 0
+        }
+    }
+}
\ No newline at end of file