diff --git a/tests/phpunit/Unit/Modules/MediaLibrary/AdminTest.php b/tests/phpunit/Unit/Modules/MediaLibrary/AdminTest.php
new file mode 100644
index 0000000..32254ed
--- /dev/null
+++ b/tests/phpunit/Unit/Modules/MediaLibrary/AdminTest.php
@@ -0,0 +1,178 @@
+<?php
+/**
+ * Tests for the MediaLibrary\Admin class.
+ *
+ * @package OneMedia\Tests\Unit\Modules\MediaLibrary
+ */
+
+declare( strict_types = 1 );
+
+namespace OneMedia\Tests\Unit\Modules\MediaLibrary;
+
+use OneMedia\Modules\MediaLibrary\Admin;
+use OneMedia\Modules\MediaSharing\Attachment;
+use OneMedia\Tests\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Tests for the MediaLibrary\Admin class.
+ */
+#[CoversClass( Admin::class )]
+final class AdminTest extends TestCase {
+	/**
+	 * Original $_REQUEST state, restored after each test.
+	 *
+	 * @var array<string, mixed>
+	 */
+	private array $original_request;
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->original_request = $_REQUEST; // phpcs:ignore WordPress.Security.NonceVerification.Recommended
+	}
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function tearDown(): void {
+		$_REQUEST = $this->original_request; // phpcs:ignore WordPress.Security.NonceVerification.Recommended
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Tests no errors on hook registration.
+	 */
+	public function test_register_hooks_adds_expected_hooks(): void {
+		$admin = new Admin();
+		$admin->register_hooks();
+
+		$this->assertTrue( true );
+	}
+
+	/**
+	 * Tests filter_ajax_query_attachments_args returns unchanged when meta_query is already set.
+	 */
+	public function test_filter_ajax_query_attachments_args_passthrough_when_meta_query_present(): void {
+		$admin = new Admin();
+		$query = [
+			'meta_query' => [ // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_query
+				[
+					'key'     => 'custom_key',
+					'value'   => '1',
+					'compare' => '=',
+				],
+			],
+		];
+
+		$this->assertSame( $query, $admin->filter_ajax_query_attachments_args( $query ) );
+	}
+
+	/**
+	 * Tests filter_ajax_query_attachments_args returns unchanged when no sync filter is in the request.
+	 */
+	public function test_filter_ajax_query_attachments_args_passthrough_without_sync_filter(): void {
+		$admin = new Admin();
+		$query = [ 'post_type' => 'attachment' ];
+
+		$this->assertSame( $query, $admin->filter_ajax_query_attachments_args( $query ) );
+	}
+
+	/**
+	 * Tests filter_ajax_query_attachments_args adds sync meta_query for onemedia_sync_status = sync.
+	 */
+	public function test_filter_ajax_query_attachments_args_adds_meta_query_for_sync_status(): void {
+		$_REQUEST['query'] = [ 'onemedia_sync_status' => Attachment::SYNC_STATUS_SYNC ]; // phpcs:ignore WordPress.Security.NonceVerification.Recommended
+
+		$admin  = new Admin();
+		$result = $admin->filter_ajax_query_attachments_args( [ 'post_type' => 'attachment' ] );
+
+		$this->assertSame(
+			[
+				[
+					'key'     => Attachment::IS_SYNC_POSTMETA_KEY,
+					'value'   => '1',
+					'compare' => '=',
+				],
+			],
+			$result['meta_query'] // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_query
+		);
+	}
+
+	/**
+	 * Tests filter_ajax_query_attachments_args adds no_sync meta_query for onemedia_sync_status = no_sync.
+	 */
+	public function test_filter_ajax_query_attachments_args_adds_meta_query_for_no_sync_status(): void {
+		$_REQUEST['query'] = [ 'onemedia_sync_status' => Attachment::SYNC_STATUS_NO_SYNC ]; // phpcs:ignore WordPress.Security.NonceVerification.Recommended
+
+		$admin  = new Admin();
+		$result = $admin->filter_ajax_query_attachments_args( [ 'post_type' => 'attachment' ] );
+
+		$this->assertSame(
+			[
+				'relation' => 'OR',
+				[
+					'key'     => Attachment::IS_SYNC_POSTMETA_KEY,
+					'value'   => '0',
+					'compare' => '=',
+				],
+				[
+					'key'     => Attachment::IS_SYNC_POSTMETA_KEY,
+					'compare' => 'NOT EXISTS',
+				],
+			],
+			$result['meta_query'] // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_query
+		);
+	}
+
+	/**
+	 * Tests filter_ajax_query_attachments_args adds sync meta_query for is_onemedia_sync = true.
+	 */
+	public function test_filter_ajax_query_attachments_args_adds_meta_query_for_is_onemedia_sync_true(): void {
+		$_REQUEST['query'] = [ 'is_onemedia_sync' => 'true' ]; // phpcs:ignore WordPress.Security.NonceVerification.Recommended
+
+		$admin  = new Admin();
+		$result = $admin->filter_ajax_query_attachments_args( [ 'post_type' => 'attachment' ] );
+
+		$this->assertSame(
+			[
+				[
+					'key'     => Attachment::IS_SYNC_POSTMETA_KEY,
+					'value'   => '1',
+					'compare' => '=',
+				],
+			],
+			$result['meta_query'] // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_query
+		);
+	}
+
+	/**
+	 * Tests filter_ajax_query_attachments_args adds no_sync meta_query for is_onemedia_sync = false.
+	 */
+	public function test_filter_ajax_query_attachments_args_adds_meta_query_for_is_onemedia_sync_false(): void {
+		$_REQUEST['query'] = [ 'is_onemedia_sync' => 'false' ]; // phpcs:ignore WordPress.Security.NonceVerification.Recommended
+
+		$admin  = new Admin();
+		$result = $admin->filter_ajax_query_attachments_args( [ 'post_type' => 'attachment' ] );
+
+		$this->assertSame(
+			[
+				'relation' => 'OR',
+				[
+					'key'     => Attachment::IS_SYNC_POSTMETA_KEY,
+					'value'   => '0',
+					'compare' => '=',
+				],
+				[
+					'key'     => Attachment::IS_SYNC_POSTMETA_KEY,
+					'compare' => 'NOT EXISTS',
+				],
+			],
+			$result['meta_query'] // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_query
+		);
+	}
+}
diff --git a/tests/phpunit/Unit/Modules/MediaLibrary/ConsumerAdminTest.php b/tests/phpunit/Unit/Modules/MediaLibrary/ConsumerAdminTest.php
new file mode 100644
index 0000000..27184f1
--- /dev/null
+++ b/tests/phpunit/Unit/Modules/MediaLibrary/ConsumerAdminTest.php
@@ -0,0 +1,103 @@
+<?php
+/**
+ * Tests for the MediaLibrary\ConsumerAdmin class.
+ *
+ * @package OneMedia\Tests\Unit\Modules\MediaLibrary
+ */
+
+declare( strict_types = 1 );
+
+namespace OneMedia\Tests\Unit\Modules\MediaLibrary;
+
+use OneMedia\Modules\MediaLibrary\ConsumerAdmin;
+use OneMedia\Modules\MediaSharing\Attachment;
+use OneMedia\Modules\Settings\Settings;
+use OneMedia\Tests\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Tests for the MediaLibrary\ConsumerAdmin class.
+ */
+#[CoversClass( ConsumerAdmin::class )]
+final class ConsumerAdminTest extends TestCase {
+	/**
+	 * Attachment post for testing.
+	 *
+	 * @var \WP_Post
+	 */
+	private \WP_Post $attachment;
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->attachment = get_post( self::factory()->attachment->create() );
+	}
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function tearDown(): void {
+		delete_option( Settings::OPTION_SITE_TYPE );
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Tests no errors on hook registration when not on a governing site.
+	 */
+	public function test_register_hooks_adds_expected_hooks(): void {
+		$consumer_admin = new ConsumerAdmin();
+		$consumer_admin->register_hooks();
+
+		$this->assertTrue( true );
+	}
+
+	/**
+	 * Tests register_hooks skips all hooks when on a governing site.
+	 */
+	public function test_register_hooks_skips_hooks_on_governing_site(): void {
+		update_option( Settings::OPTION_SITE_TYPE, Settings::SITE_TYPE_GOVERNING );
+
+		$consumer_admin = new ConsumerAdmin();
+		$consumer_admin->register_hooks();
+
+		$this->assertFalse( has_filter( 'delete_attachment', [ $consumer_admin, 'prevent_attachment_deletion' ] ) );
+	}
+
+	/**
+	 * Tests remove_edit_delete_links removes edit and delete actions for a synced attachment.
+	 */
+	public function test_remove_edit_delete_links_removes_actions_for_synced(): void {
+		Attachment::set_is_synced( $this->attachment->ID, true );
+
+		$consumer_admin = new ConsumerAdmin();
+		$result         = $consumer_admin->remove_edit_delete_links(
+			[
+				'edit'   => '<a>Edit</a>',
+				'delete' => '<a>Delete</a>',
+			],
+			$this->attachment
+		);
+
+		$this->assertArrayNotHasKey( 'edit', $result );
+		$this->assertArrayNotHasKey( 'delete', $result );
+	}
+
+	/**
+	 * Tests remove_edit_delete_links keeps all actions for a non-synced attachment.
+	 */
+	public function test_remove_edit_delete_links_keeps_actions_for_non_synced(): void {
+		$consumer_admin = new ConsumerAdmin();
+		$actions        = [
+			'edit'   => '<a>Edit</a>',
+			'delete' => '<a>Delete</a>',
+		];
+
+		$result = $consumer_admin->remove_edit_delete_links( $actions, $this->attachment );
+
+		$this->assertSame( $actions, $result );
+	}
+}
diff --git a/tests/phpunit/Unit/Modules/MediaSharing/AttachmentTest.php b/tests/phpunit/Unit/Modules/MediaSharing/AttachmentTest.php
new file mode 100644
index 0000000..225fa90
--- /dev/null
+++ b/tests/phpunit/Unit/Modules/MediaSharing/AttachmentTest.php
@@ -0,0 +1,108 @@
+<?php
+/**
+ * Tests for the MediaSharing\Attachment class.
+ *
+ * @package OneMedia\Tests\Unit\Modules\MediaSharing
+ */
+
+declare( strict_types = 1 );
+
+namespace OneMedia\Tests\Unit\Modules\MediaSharing;
+
+use OneMedia\Modules\MediaSharing\Attachment;
+use OneMedia\Modules\Settings\Settings;
+use OneMedia\Tests\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Tests for the MediaSharing\Attachment class.
+ */
+#[CoversClass( Attachment::class )]
+final class AttachmentTest extends TestCase {
+	/**
+	 * Attachment ID for testing.
+	 *
+	 * @var int
+	 */
+	private int $attachment_id;
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->attachment_id = self::factory()->attachment->create();
+	}
+
+	/**
+	 * Tests no errors on hook registration.
+	 */
+	public function test_register_hooks_adds_expected_hooks(): void {
+		$attachment = new Attachment();
+		$attachment->register_hooks();
+
+		$this->assertTrue( true );
+	}
+
+	/**
+	 * Tests is_sync_attachment returns false for a new attachment with no meta.
+	 */
+	public function test_is_sync_attachment_returns_false_for_new_attachment(): void {
+		$this->assertFalse( Attachment::is_sync_attachment( $this->attachment_id ) );
+	}
+
+	/**
+	 * Tests set_is_synced and is_sync_attachment round-trip.
+	 */
+	public function test_set_is_synced_and_is_sync_attachment(): void {
+		Attachment::set_is_synced( $this->attachment_id, true );
+		$this->assertTrue( Attachment::is_sync_attachment( $this->attachment_id ) );
+
+		Attachment::set_is_synced( $this->attachment_id, false );
+		$this->assertFalse( Attachment::is_sync_attachment( $this->attachment_id ) );
+	}
+
+	/**
+	 * Tests get_sync_sites returns empty array when not on a governing site.
+	 */
+	public function test_get_sync_sites_returns_empty_when_not_governing(): void {
+		$this->assertSame( [], Attachment::get_sync_sites( $this->attachment_id ) );
+	}
+
+	/**
+	 * Tests get_sync_sites returns empty array on governing site when no meta is stored.
+	 */
+	public function test_get_sync_sites_returns_empty_on_governing_when_no_meta(): void {
+		update_option( Settings::OPTION_SITE_TYPE, Settings::SITE_TYPE_GOVERNING );
+
+		$this->assertSame( [], Attachment::get_sync_sites( $this->attachment_id ) );
+
+		delete_option( Settings::OPTION_SITE_TYPE );
+	}
+
+	/**
+	 * Tests update_sync_attachment_versions and get_sync_attachment_versions round-trip.
+	 */
+	public function test_update_and_get_sync_attachment_versions(): void {
+		$versions = [
+			[
+				'last_used' => 1000000,
+				'file'      => [
+					'path' => '/var/www/html/wp-content/uploads/test.jpg',
+					'url'  => 'https://example.com/wp-content/uploads/test.jpg',
+				],
+			],
+		];
+
+		$this->assertTrue( Attachment::update_sync_attachment_versions( $this->attachment_id, $versions ) );
+		$this->assertSame( $versions, Attachment::get_sync_attachment_versions( $this->attachment_id ) );
+	}
+
+	/**
+	 * Tests get_sync_attachment_versions returns empty array when no versions are stored.
+	 */
+	public function test_get_sync_attachment_versions_returns_empty_when_not_set(): void {
+		$this->assertSame( [], Attachment::get_sync_attachment_versions( $this->attachment_id ) );
+	}
+}
diff --git a/tests/phpunit/Unit/Modules/MediaSharing/MediaProtectionTest.php b/tests/phpunit/Unit/Modules/MediaSharing/MediaProtectionTest.php
new file mode 100644
index 0000000..30144e8
--- /dev/null
+++ b/tests/phpunit/Unit/Modules/MediaSharing/MediaProtectionTest.php
@@ -0,0 +1,100 @@
+<?php
+/**
+ * Tests for the MediaSharing\MediaProtection class.
+ *
+ * @package OneMedia\Tests\Unit\Modules\MediaSharing
+ */
+
+declare( strict_types = 1 );
+
+namespace OneMedia\Tests\Unit\Modules\MediaSharing;
+
+use OneMedia\Modules\MediaSharing\Attachment;
+use OneMedia\Modules\MediaSharing\MediaProtection;
+use OneMedia\Tests\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Tests for the MediaSharing\MediaProtection class.
+ */
+#[CoversClass( MediaProtection::class )]
+final class MediaProtectionTest extends TestCase {
+	/**
+	 * Attachment ID for testing.
+	 *
+	 * @var int
+	 */
+	private int $attachment_id;
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->attachment_id = self::factory()->attachment->create();
+	}
+
+	/**
+	 * Tests no errors on hook registration.
+	 */
+	public function test_register_hooks_adds_expected_hooks(): void {
+		$protection = new MediaProtection();
+		$protection->register_hooks();
+
+		$this->assertTrue( true );
+	}
+
+	/**
+	 * Tests prevent_sync_media_editing returns do_not_allow for a synced attachment on edit_post.
+	 */
+	public function test_prevent_sync_media_editing_returns_do_not_allow_for_synced(): void {
+		Attachment::set_is_synced( $this->attachment_id, true );
+
+		$protection = new MediaProtection();
+		$result     = $protection->prevent_sync_media_editing(
+			[ 'edit_posts' ],
+			'edit_post',
+			1,
+			[ $this->attachment_id ]
+		);
+
+		$this->assertSame( [ 'do_not_allow' ], $result );
+	}
+
+	/**
+	 * Tests prevent_sync_media_editing passes through caps for a non-synced attachment.
+	 */
+	public function test_prevent_sync_media_editing_passes_through_for_non_synced(): void {
+		$caps       = [ 'edit_posts' ];
+		$protection = new MediaProtection();
+
+		$result = $protection->prevent_sync_media_editing(
+			$caps,
+			'edit_post',
+			1,
+			[ $this->attachment_id ]
+		);
+
+		$this->assertSame( $caps, $result );
+	}
+
+	/**
+	 * Tests prevent_sync_media_editing passes through caps for non-edit capabilities.
+	 */
+	public function test_prevent_sync_media_editing_passes_through_for_non_edit_cap(): void {
+		Attachment::set_is_synced( $this->attachment_id, true );
+
+		$caps       = [ 'upload_files' ];
+		$protection = new MediaProtection();
+
+		$result = $protection->prevent_sync_media_editing(
+			$caps,
+			'upload_files',
+			1,
+			[ $this->attachment_id ]
+		);
+
+		$this->assertSame( $caps, $result );
+	}
+}
diff --git a/tests/phpunit/Unit/Modules/MediaSharing/UserInterfaceTest.php b/tests/phpunit/Unit/Modules/MediaSharing/UserInterfaceTest.php
new file mode 100644
index 0000000..5940e1f
--- /dev/null
+++ b/tests/phpunit/Unit/Modules/MediaSharing/UserInterfaceTest.php
@@ -0,0 +1,96 @@
+<?php
+/**
+ * Tests for the MediaSharing\UserInterface class.
+ *
+ * @package OneMedia\Tests\Unit\Modules\MediaSharing
+ */
+
+declare( strict_types = 1 );
+
+namespace OneMedia\Tests\Unit\Modules\MediaSharing;
+
+use OneMedia\Modules\MediaSharing\Attachment;
+use OneMedia\Modules\MediaSharing\UserInterface;
+use OneMedia\Tests\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Tests for the MediaSharing\UserInterface class.
+ */
+#[CoversClass( UserInterface::class )]
+final class UserInterfaceTest extends TestCase {
+	/**
+	 * Attachment post for testing.
+	 *
+	 * @var \WP_Post
+	 */
+	private \WP_Post $attachment;
+
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->attachment = get_post( self::factory()->attachment->create() );
+	}
+
+	/**
+	 * Tests no errors on hook registration.
+	 */
+	public function test_register_hooks_adds_expected_hooks(): void {
+		$ui = new UserInterface();
+		$ui->register_hooks();
+
+		$this->assertTrue( true );
+	}
+
+	/**
+	 * Tests add_sync_column appends the sync status column.
+	 */
+	public function test_add_sync_column_appends_column(): void {
+		$ui = new UserInterface();
+
+		$result = $ui->add_sync_column(
+			[
+				'title' => 'Title',
+				'date'  => 'Date',
+			]
+		);
+
+		$this->assertArrayHasKey( 'onemedia_sync_status', $result );
+	}
+
+	/**
+	 * Tests filter_media_row_actions removes delete action for synced attachment.
+	 */
+	public function test_filter_media_row_actions_removes_delete_for_synced(): void {
+		Attachment::set_is_synced( $this->attachment->ID, true );
+
+		$ui     = new UserInterface();
+		$result = $ui->filter_media_row_actions(
+			[
+				'edit'   => '<a>Edit</a>',
+				'delete' => '<a>Delete</a>',
+			],
+			$this->attachment
+		);
+
+		$this->assertArrayNotHasKey( 'delete', $result );
+	}
+
+	/**
+	 * Tests filter_media_row_actions keeps all actions for non-synced attachment.
+	 */
+	public function test_filter_media_row_actions_keeps_actions_for_non_synced(): void {
+		$ui      = new UserInterface();
+		$actions = [
+			'edit'   => '<a>Edit</a>',
+			'delete' => '<a>Delete</a>',
+		];
+
+		$result = $ui->filter_media_row_actions( $actions, $this->attachment );
+
+		$this->assertSame( $actions, $result );
+	}
+}
diff --git a/tests/phpunit/Unit/Modules/Settings/AdminTest.php b/tests/phpunit/Unit/Modules/Settings/AdminTest.php
new file mode 100644
index 0000000..8ecf959
--- /dev/null
+++ b/tests/phpunit/Unit/Modules/Settings/AdminTest.php
@@ -0,0 +1,77 @@
+<?php
+/**
+ * Tests for the Settings\Admin class.
+ *
+ * @package OneMedia\Tests\Unit\Modules\Settings
+ */
+
+declare( strict_types = 1 );
+
+namespace OneMedia\Tests\Unit\Modules\Settings;
+
+use OneMedia\Modules\Settings\Admin;
+use OneMedia\Tests\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Tests for the Settings\Admin class.
+ */
+#[CoversClass( Admin::class )]
+final class AdminTest extends TestCase {
+	/**
+	 * Tests no errors on class instantiation or hook registration.
+	 */
+	public function test_register_hooks_adds_expected_hooks(): void {
+		$admin = new Admin();
+		$admin->register_hooks();
+
+		$this->assertTrue( true );
+	}
+
+	/**
+	 * Tests screen_callback outputs the settings page mount point.
+	 */
+	public function test_screen_callback_outputs_expected_html(): void {
+		$admin = new Admin();
+
+		ob_start();
+		$admin->screen_callback();
+		$output = ob_get_clean();
+
+		$this->assertStringContainsString( 'onemedia-settings-page', (string) $output );
+	}
+
+	/**
+	 * Tests add_action_links appends a settings link to an empty array.
+	 */
+	public function test_add_action_links_appends_settings_link(): void {
+		$admin = new Admin();
+
+		$result = $admin->add_action_links( [] );
+
+		$this->assertCount( 1, $result );
+		$this->assertStringContainsString( 'Settings', $result[0] );
+	}
+
+	/**
+	 * Tests add_action_links preserves existing links.
+	 */
+	public function test_add_action_links_preserves_existing_links(): void {
+		$admin = new Admin();
+
+		$result = $admin->add_action_links( [ '<a href="#">Existing</a>' ] );
+
+		$this->assertCount( 2, $result );
+	}
+
+	/**
+	 * Tests add_body_classes returns a string.
+	 */
+	public function test_add_body_classes_returns_string(): void {
+		$admin = new Admin();
+
+		$result = $admin->add_body_classes( 'existing-class' );
+
+		$this->assertIsString( $result );
+	}
+}