diff --git a/core/data/src/test/kotlin/com/google/samples/apps/nowinandroid/core/data/util/StateErrorMonitorTest.kt b/core/data/src/test/kotlin/com/google/samples/apps/nowinandroid/core/data/util/StateErrorMonitorTest.kt
new file mode 100644
index 000000000..3e928cea7
--- /dev/null
+++ b/core/data/src/test/kotlin/com/google/samples/apps/nowinandroid/core/data/util/StateErrorMonitorTest.kt
@@ -0,0 +1,147 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import com.google.samples.apps.nowinandroid.core.data.util.StateErrorMonitor
+import com.google.samples.apps.nowinandroid.core.model.data.MessageData
+import com.google.samples.apps.nowinandroid.core.model.data.MessageType
+import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.UnconfinedTestDispatcher
+import kotlinx.coroutines.test.runTest
+import org.junit.Before
+import org.junit.Test
+import kotlin.test.assertEquals
+
+/**
+ * Unit tests for [StateErrorMonitor].
+ */
+
+class StateErrorMonitorTest {
+
+    // Subject under test.
+    private lateinit var state: StateErrorMonitor
+
+    private lateinit var messages: List<MessageData?>
+
+    private val testString = "Test Error Message"
+    private val testData = MessageData(MessageType.MESSAGE("Test Error Message 1"))
+    private val testData2 = MessageData(MessageType.MESSAGE("Test Error Message 2"))
+
+    @Before
+    fun setup() {
+        state = StateErrorMonitor()
+        messages = emptyList()
+    }
+
+    @Test
+    fun whenErrorIsNotAdded_NullIsPresent() = runTest(UnconfinedTestDispatcher()) {
+        backgroundScope.launch {
+            state.messages.collect {
+                messages = it
+            }
+        }
+        assertEquals(
+            emptyList(),
+            messages,
+        )
+    }
+
+    @Test
+    fun whenErrorIsAddedByString_ErrorMessageIsPresent() = runTest(UnconfinedTestDispatcher()) {
+        backgroundScope.launch {
+            state.messages.collect {
+                messages = it
+            }
+        }
+
+        val expect = state.addMessageByString(testString)
+
+        assertEquals(
+            expect,
+            messages.firstOrNull(),
+        )
+    }
+
+    @Test
+    fun whenErrorIsAddedByData_ErrorMessageIsPresent() = runTest(UnconfinedTestDispatcher()) {
+        backgroundScope.launch {
+            state.messages.collect {
+                messages = it
+            }
+        }
+
+        state.addMessageByData(testData)
+
+        assertEquals(
+            testData,
+            messages.firstOrNull(),
+        )
+    }
+
+    @Test
+    fun whenErrorsAreAdded_FirstErrorMessageIsPresent() =
+        runTest(UnconfinedTestDispatcher()) {
+            state.addMessageByData(testData)
+            state.addMessageByString(testString)
+
+            backgroundScope.launch {
+                state.messages.collect {
+                    messages = it
+                }
+            }
+
+            assertEquals(
+                testData,
+                messages.firstOrNull(),
+            )
+        }
+
+    @Test
+    fun whenErrorIsCleared_ErrorMessageIsNotPresent() =
+        runTest(UnconfinedTestDispatcher()) {
+            backgroundScope.launch {
+                state.messages.collect {
+                    messages = it
+                }
+            }
+            state.addMessageByData(testData)
+            state.clearMessage(testData)
+
+            assertEquals(
+                emptyList(),
+                messages,
+            )
+        }
+
+    @Test
+    fun whenErrorsAreCleared_NextErrorMessageIsPresent() =
+        runTest(UnconfinedTestDispatcher()) {
+            backgroundScope.launch {
+                state.messages.collect {
+                    messages = it
+                }
+            }
+            state.addMessageByData(testData)
+            state.addMessageByData(testData2)
+
+            state.clearMessage(testData)
+
+            assertEquals(
+                testData2,
+                messages.firstOrNull(),
+            )
+        }
+}