diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index faf15e96d..66eaceb81 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -17,6 +17,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          submodules: true
 
       - name: Set up JDK for running Gradle
         uses: actions/setup-java@v4
@@ -46,6 +48,8 @@ jobs:
       fail-fast: false
     steps:
       - uses: actions/checkout@v4
+        with:
+          submodules: true
 
       - id: setup-test-java
         name: Set up JDK ${{ matrix.test-java-version }} for running tests
@@ -78,6 +82,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          submodules: true
 
       - name: Set up JDK for running Gradle
         uses: actions/setup-java@v4
@@ -131,6 +137,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          submodules: true
 
       - name: Set up JDK for running Gradle
         uses: actions/setup-java@v4
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 000000000..1db07bfa3
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "opamp-client/opamp-spec"]
+	path = opamp-client/opamp-spec
+	url = git@github.com:open-telemetry/opamp-spec.git
diff --git a/opamp-client/build.gradle.kts b/opamp-client/build.gradle.kts
index c094faf03..1716d4051 100644
--- a/opamp-client/build.gradle.kts
+++ b/opamp-client/build.gradle.kts
@@ -1,5 +1,6 @@
 plugins {
   id("otel.java-conventions")
+  id("com.google.protobuf") version "0.9.4"
 }
 
 description = "Client implementation of the OpAMP spec."
@@ -9,3 +10,33 @@ java {
   sourceCompatibility = JavaVersion.VERSION_1_8
   targetCompatibility = JavaVersion.VERSION_1_8
 }
+
+sourceSets {
+  main {
+    proto {
+      srcDir("opamp-spec/proto")
+    }
+  }
+}
+
+tasks.withType<JavaCompile>().configureEach {
+  with(options) {
+    // Suppressing warnings about the usage of deprecated methods.
+    // This is needed because the Protobuf plugin (com.google.protobuf) generates code that uses deprecated methods.
+    compilerArgs.add("-Xlint:-deprecation")
+  }
+}
+
+val protobufVersion = "4.28.2"
+
+protobuf {
+  protoc {
+    artifact = "com.google.protobuf:protoc:$protobufVersion"
+  }
+}
+
+dependencies {
+  implementation("com.google.protobuf:protobuf-java:$protobufVersion")
+  annotationProcessor("com.google.auto.value:auto-value")
+  compileOnly("com.google.auto.value:auto-value-annotations")
+}
diff --git a/opamp-client/opamp-spec b/opamp-client/opamp-spec
new file mode 160000
index 000000000..e74e4fc49
--- /dev/null
+++ b/opamp-client/opamp-spec
@@ -0,0 +1 @@
+Subproject commit e74e4fc498fd1dc06e2e7368966c1ce87ee3fa9b
diff --git a/opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/OpampClient.java b/opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/OpampClient.java
new file mode 100644
index 000000000..32ed77656
--- /dev/null
+++ b/opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/OpampClient.java
@@ -0,0 +1,101 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.opamp.client.internal;
+
+import io.opentelemetry.opamp.client.internal.response.MessageData;
+import opamp.proto.Opamp;
+
+public interface OpampClient {
+
+  /**
+   * Starts the client and begin attempts to connect to the Server. Once connection is established
+   * the client will attempt to maintain it by reconnecting if the connection is lost. All failed
+   * connection attempts will be reported via {@link Callbacks#onConnectFailed(OpampClient,
+   * Throwable)} callback.
+   *
+   * <p>This method does not wait until the connection to the Server is established and will likely
+   * return before the connection attempts are even made.
+   *
+   * <p>This method may be called only once.
+   *
+   * @param callbacks The Callback to which the Client will notify about any Server requests and
+   *     responses.
+   */
+  void start(Callbacks callbacks);
+
+  /**
+   * Stops the client. May be called only after {@link #start(Callbacks)}. May be called only once.
+   * After this call returns successfully it is guaranteed that no callbacks will be called. Once
+   * stopped, the client cannot be started again.
+   */
+  void stop();
+
+  /**
+   * Sets attributes of the Agent. The attributes will be included in the next status report sent to
+   * the Server. When called after {@link #start(Callbacks)}, the attributes will be included in the
+   * next outgoing status report. This is typically used by Agents which allow their
+   * AgentDescription to change dynamically while the OpAMPClient is started. May be also called
+   * from {@link Callbacks#onMessage(OpampClient, MessageData)}.
+   *
+   * @param agentDescription The new agent description.
+   */
+  void setAgentDescription(Opamp.AgentDescription agentDescription);
+
+  /**
+   * Sets the current remote config status which will be sent in the next agent to server request.
+   *
+   * @param remoteConfigStatus The new remote config status.
+   */
+  void setRemoteConfigStatus(Opamp.RemoteConfigStatus remoteConfigStatus);
+
+  interface Callbacks {
+    /**
+     * Called when the connection is successfully established to the Server. May be called after
+     * {@link #start(Callbacks)} is called and every time a connection is established to the Server.
+     * For WebSocket clients this is called after the handshake is completed without any error. For
+     * HTTP clients this is called for any request if the response status is OK.
+     *
+     * @param client The relevant {@link OpampClient} instance.
+     */
+    void onConnect(OpampClient client);
+
+    /**
+     * Called when the connection to the Server cannot be established. May be called after {@link
+     * #start(Callbacks)} is called and tries to connect to the Server. May also be called if the
+     * connection is lost and reconnection attempt fails.
+     *
+     * @param client The relevant {@link OpampClient} instance.
+     * @param throwable The exception.
+     */
+    void onConnectFailed(OpampClient client, Throwable throwable);
+
+    /**
+     * Called when the Server reports an error in response to some previously sent request. Useful
+     * for logging purposes. The Agent should not attempt to process the error by reconnecting or
+     * retrying previous operations. The client handles the ErrorResponse_UNAVAILABLE case
+     * internally by performing retries as necessary.
+     *
+     * @param client The relevant {@link OpampClient} instance.
+     * @param errorResponse The error returned by the Server.
+     */
+    void onErrorResponse(OpampClient client, Opamp.ServerErrorResponse errorResponse);
+
+    /**
+     * Called when the Agent receives a message that needs processing. See {@link MessageData}
+     * definition for the data that may be available for processing. During onMessage execution the
+     * {@link OpampClient} functions that change the status of the client may be called, e.g. if
+     * RemoteConfig is processed then {@link
+     * #setRemoteConfigStatus(opamp.proto.Opamp.RemoteConfigStatus)} should be called to reflect the
+     * processing result. These functions may also be called after onMessage returns. This is
+     * advisable if processing can take a long time. In that case returning quickly is preferable to
+     * avoid blocking the {@link OpampClient}.
+     *
+     * @param client The relevant {@link OpampClient} instance.
+     * @param messageData The server response data that needs processing.
+     */
+    void onMessage(OpampClient client, MessageData messageData);
+  }
+}
diff --git a/opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/response/MessageData.java b/opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/response/MessageData.java
new file mode 100644
index 000000000..0cd4b0fdf
--- /dev/null
+++ b/opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/response/MessageData.java
@@ -0,0 +1,32 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.opamp.client.internal.response;
+
+import com.google.auto.value.AutoValue;
+import io.opentelemetry.opamp.client.internal.OpampClient;
+import javax.annotation.Nullable;
+import opamp.proto.Opamp;
+
+/**
+ * Data class provided in {@link OpampClient.Callbacks#onMessage(OpampClient, MessageData)} with
+ * Server's provided status changes.
+ */
+@AutoValue
+public abstract class MessageData {
+  @Nullable
+  public abstract Opamp.AgentRemoteConfig getRemoteConfig();
+
+  public static Builder builder() {
+    return new AutoValue_MessageData.Builder();
+  }
+
+  @AutoValue.Builder
+  public abstract static class Builder {
+    public abstract Builder setRemoteConfig(Opamp.AgentRemoteConfig remoteConfig);
+
+    public abstract MessageData build();
+  }
+}