diff --git a/internal/base/BUILD b/internal/base/BUILD
index 69faa567..a7cc6631 100644
--- a/internal/base/BUILD
+++ b/internal/base/BUILD
@@ -13,6 +13,7 @@ cc_library(
     visibility = [
         "//fastpair:__subpackages__",
         "//internal:__pkg__",
+        "//internal/platform:__pkg__",
         "//location/nearby/cpp/fastpair:__subpackages__",
         "//location/nearby/cpp/sharing:__subpackages__",
     ],
diff --git a/internal/platform/BUILD b/internal/platform/BUILD
index b6508e38..b3479b25 100644
--- a/internal/platform/BUILD
+++ b/internal/platform/BUILD
@@ -409,9 +409,11 @@ cc_library(
         ":logging",
         ":types",
         ":uuid",
+        "//internal/base",
         "//internal/platform/implementation:comm",
         "//internal/platform/implementation:platform",
         "@com_google_absl//absl/container:flat_hash_map",
+        "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/functional:any_invocable",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/strings",
diff --git a/internal/platform/bluetooth_classic.cc b/internal/platform/bluetooth_classic.cc
index c30fdf06..beade20d 100644
--- a/internal/platform/bluetooth_classic.cc
+++ b/internal/platform/bluetooth_classic.cc
@@ -19,7 +19,12 @@
 
 namespace nearby {
 
-BluetoothClassicMedium::~BluetoothClassicMedium() { StopDiscovery(); }
+BluetoothClassicMedium::~BluetoothClassicMedium() {
+  if (!observer_list_.empty()) {
+    impl_->RemoveObserver(this);
+  }
+  StopDiscovery();
+}
 
 BluetoothSocket BluetoothClassicMedium::ConnectToService(
     BluetoothDevice& remote_device, const std::string& service_uuid,
@@ -94,4 +99,54 @@ bool BluetoothClassicMedium::StopDiscovery() {
   return impl_->StopDiscovery();
 }
 
+void BluetoothClassicMedium::AddObserver(Observer* observer) {
+  MutexLock lock(&mutex_);
+  if (observer_list_.empty()) {
+    impl_->AddObserver(this);
+  }
+  observer_list_.AddObserver(observer);
+}
+void BluetoothClassicMedium::RemoveObserver(Observer* observer) {
+  MutexLock lock(&mutex_);
+  observer_list_.RemoveObserver(observer);
+  if (observer_list_.empty()) {
+    impl_->RemoveObserver(this);
+  }
+}
+
+// api::BluetoothClassicMedium::Observer methods
+void BluetoothClassicMedium::DeviceAdded(api::BluetoothDevice& device) {
+  BluetoothDevice bt_device(&device);
+  for (auto* observer : observer_list_.GetObservers()) {
+    observer->DeviceAdded(bt_device);
+  }
+}
+void BluetoothClassicMedium::DeviceRemoved(api::BluetoothDevice& device) {
+  BluetoothDevice bt_device(&device);
+  for (auto* observer : observer_list_.GetObservers()) {
+    observer->DeviceRemoved(bt_device);
+  }
+}
+void BluetoothClassicMedium::DeviceAddressChanged(
+    api::BluetoothDevice& device, absl::string_view old_address) {
+  BluetoothDevice bt_device(&device);
+  for (auto* observer : observer_list_.GetObservers()) {
+    observer->DeviceAddressChanged(bt_device, old_address);
+  }
+}
+void BluetoothClassicMedium::DevicePairedChanged(api::BluetoothDevice& device,
+                                                 bool new_paired_status) {
+  BluetoothDevice bt_device(&device);
+  for (auto* observer : observer_list_.GetObservers()) {
+    observer->DevicePairedChanged(bt_device, new_paired_status);
+  }
+}
+void BluetoothClassicMedium::DeviceConnectedStateChanged(
+    api::BluetoothDevice& device, bool connected) {
+  BluetoothDevice bt_device(&device);
+  for (auto* observer : observer_list_.GetObservers()) {
+    observer->DeviceConnectedStateChanged(bt_device, connected);
+  }
+}
+
 }  // namespace nearby
diff --git a/internal/platform/bluetooth_classic.h b/internal/platform/bluetooth_classic.h
index c9a8180e..319d14f5 100644
--- a/internal/platform/bluetooth_classic.h
+++ b/internal/platform/bluetooth_classic.h
@@ -19,6 +19,8 @@
 #include <string>
 
 #include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
+#include "internal/base/observer_list.h"
 #include "internal/platform/bluetooth_adapter.h"
 #include "internal/platform/byte_array.h"
 #include "internal/platform/cancellation_flag.h"
@@ -126,7 +128,8 @@ class BluetoothServerSocket final {
 
 // Container of operations that can be performed over the Bluetooth Classic
 // medium.
-class BluetoothClassicMedium final {
+class BluetoothClassicMedium final
+    : public api::BluetoothClassicMedium::Observer {
  public:
   using Platform = api::ImplementationPlatform;
   struct DiscoveryCallback {
@@ -148,11 +151,39 @@ class BluetoothClassicMedium final {
     BluetoothDevice device;
   };
 
+  class Observer {
+   public:
+    virtual ~Observer() = default;
+
+    // Called when a new `device` is added. The `device` parameter becomes
+    // invalid after the call.
+    virtual void DeviceAdded(BluetoothDevice& device) {}
+
+    // Called when `device` is removed. The `device` parameter becomes invalid
+    // after the call.
+    virtual void DeviceRemoved(BluetoothDevice& device) {}
+
+    // Called when the address of `device` changed due to pairing. The
+    // `device` parameter becomes invalid after the call.
+    virtual void DeviceAddressChanged(BluetoothDevice& device,
+                                      absl::string_view old_address) {}
+
+    // Called when the paired property of `device` changed. The `device`
+    // parameter becomes invalid after the call.
+    virtual void DevicePairedChanged(BluetoothDevice& device,
+                                     bool new_paired_status) {}
+
+    // Called when `device` has connected or disconnected. The `device`
+    // parameter becomes invalid after the call.
+    virtual void DeviceConnectedStateChanged(BluetoothDevice& device,
+                                             bool connected) {}
+  };
+
   explicit BluetoothClassicMedium(BluetoothAdapter& adapter)
       : impl_(Platform::CreateBluetoothClassicMedium(adapter.GetImpl())),
         adapter_(adapter) {}
 
-  ~BluetoothClassicMedium();
+  ~BluetoothClassicMedium() override;
 
   // NOTE(DiscoveryCallback):
   // BluetoothDevice is a proxy object created as a result of BT discovery.
@@ -217,6 +248,23 @@ class BluetoothClassicMedium final {
     return BluetoothDevice(impl_->GetRemoteDevice(mac_address));
   }
 
+  // Adds an observer. `observer` must be valid until RemoveObserver is called,
+  // or BluetoothClassicMedium is destroyed.
+  void AddObserver(Observer* observer);
+
+  // Removes an observer. It's OK to remove an unregistered observer.
+  void RemoveObserver(Observer* observer);
+
+  // api::BluetoothClassicMedium::Observer methods
+  void DeviceAdded(api::BluetoothDevice& device) override;
+  void DeviceRemoved(api::BluetoothDevice& device) override;
+  void DeviceAddressChanged(api::BluetoothDevice& device,
+                            absl::string_view old_address) override;
+  void DevicePairedChanged(api::BluetoothDevice& device,
+                           bool new_paired_status) override;
+  void DeviceConnectedStateChanged(api::BluetoothDevice& device,
+                                   bool connected) override;
+
  private:
   Mutex mutex_;
   std::unique_ptr<api::BluetoothClassicMedium> impl_;
@@ -226,6 +274,7 @@ class BluetoothClassicMedium final {
       devices_ ABSL_GUARDED_BY(mutex_);
   DiscoveryCallback discovery_callback_ ABSL_GUARDED_BY(mutex_);
   bool discovery_enabled_ ABSL_GUARDED_BY(mutex_) = false;
+  ObserverList<Observer> observer_list_;
 };
 
 }  // namespace nearby
diff --git a/internal/platform/implementation/bluetooth_classic.h b/internal/platform/implementation/bluetooth_classic.h
index 5dfba4ef..7f8f9cf3 100644
--- a/internal/platform/implementation/bluetooth_classic.h
+++ b/internal/platform/implementation/bluetooth_classic.h
@@ -111,6 +111,29 @@ class BluetoothClassicMedium {
         DefaultCallback<BluetoothDevice&>();
   };
 
+  class Observer {
+   public:
+    virtual ~Observer() = default;
+
+    // Called when a new `device` is added to the adapter.
+    virtual void DeviceAdded(BluetoothDevice& device) {}
+
+    // Called when `device` is removed from the adapter.
+    virtual void DeviceRemoved(BluetoothDevice& device) {}
+
+    // Called when the address of `device` changed due to pairing.
+    virtual void DeviceAddressChanged(BluetoothDevice& device,
+                                      absl::string_view old_address) {}
+
+    // Called when the paired property of `device` changed.
+    virtual void DevicePairedChanged(BluetoothDevice& device,
+                                     bool new_paired_status) {}
+
+    // Called when `device` has connected or disconnected.
+    virtual void DeviceConnectedStateChanged(BluetoothDevice& device,
+                                             bool connected) {}
+  };
+
   // https://developer.android.com/reference/android/bluetooth/BluetoothAdapter.html#startDiscovery()
   //
   // Returns true once the process of discovery has been initiated.
@@ -152,6 +175,9 @@ class BluetoothClassicMedium {
       const std::string& service_name, const std::string& service_uuid) = 0;
 
   virtual BluetoothDevice* GetRemoteDevice(const std::string& mac_address) = 0;
+
+  virtual void AddObserver(Observer* observer) = 0;
+  virtual void RemoveObserver(Observer* observer) = 0;
 };
 
 }  // namespace api
diff --git a/internal/platform/implementation/g3/bluetooth_classic.cc b/internal/platform/implementation/g3/bluetooth_classic.cc
index a8d2c93b..b8544aba 100644
--- a/internal/platform/implementation/g3/bluetooth_classic.cc
+++ b/internal/platform/implementation/g3/bluetooth_classic.cc
@@ -277,5 +277,14 @@ api::BluetoothDevice* BluetoothClassicMedium::GetRemoteDevice(
   return env.FindBluetoothDevice(mac_address);
 }
 
+void BluetoothClassicMedium::AddObserver(
+    api::BluetoothClassicMedium::Observer* observer) {
+  // TODO(b/269521993): Implement observer callbacks.
+}
+void BluetoothClassicMedium::RemoveObserver(
+    api::BluetoothClassicMedium::Observer* observer) {
+  // TODO(b/269521993): Implement observer callbacks.
+}
+
 }  // namespace g3
 }  // namespace nearby
diff --git a/internal/platform/implementation/g3/bluetooth_classic.h b/internal/platform/implementation/g3/bluetooth_classic.h
index 668f8cec..1db7af17 100644
--- a/internal/platform/implementation/g3/bluetooth_classic.h
+++ b/internal/platform/implementation/g3/bluetooth_classic.h
@@ -236,6 +236,9 @@ class BluetoothClassicMedium : public api::BluetoothClassicMedium {
   api::BluetoothDevice* GetRemoteDevice(
       const std::string& mac_address) override;
 
+  void AddObserver(Observer* observer) override;
+  void RemoveObserver(Observer* observer) override;
+
  private:
   absl::Mutex mutex_;
   BluetoothAdapter* adapter_;  // Our device adapter; read-only.
diff --git a/internal/platform/implementation/windows/bluetooth_classic_medium.h b/internal/platform/implementation/windows/bluetooth_classic_medium.h
index 35b6ba6f..a9f2b4ef 100644
--- a/internal/platform/implementation/windows/bluetooth_classic_medium.h
+++ b/internal/platform/implementation/windows/bluetooth_classic_medium.h
@@ -148,6 +148,15 @@ class BluetoothClassicMedium : public api::BluetoothClassicMedium {
   api::BluetoothDevice* GetRemoteDevice(const std::string& mac_address) override
       ABSL_LOCKS_EXCLUDED(mutex_);
 
+  void AddObserver(Observer* observer) override {
+    // TODO(b/269521993): Implement.
+  }
+
+  // Removes an observer. It's OK to remove an unregistered observer.
+  void RemoveObserver(Observer* observer) override {
+    // TODO(b/269521993): Implement.
+  }
+
  private:
   bool StartScanning() ABSL_SHARED_LOCKS_REQUIRED(mutex_);
   bool StopScanning() ABSL_SHARED_LOCKS_REQUIRED(mutex_);