diff --git a/src/TwigTweakExtension.php b/src/TwigTweakExtension.php
index 7052240..b23ec21 100644
--- a/src/TwigTweakExtension.php
+++ b/src/TwigTweakExtension.php
@@ -6,6 +6,7 @@ use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\Unicode;
 use Drupal\Component\Uuid\Uuid;
 use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Field\EntityReferenceFieldItemListInterface;
 use Drupal\Core\Field\FieldItemInterface;
 use Drupal\Core\Field\FieldItemListInterface;
@@ -14,6 +15,7 @@ use Drupal\Core\Link;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Render\Markup;
 use Drupal\Core\Site\Settings;
+use Drupal\Core\Theme\ThemeManagerInterface;
 use Drupal\Core\Url;
 use Drupal\file\Entity\File;
 use Drupal\file\FileInterface;
@@ -35,6 +37,28 @@ use Twig\TwigFunction;
  */
 class TwigTweakExtension extends AbstractExtension {
 
+  /**
+   * The module handler to invoke alter hooks.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The theme manager to invoke alter hooks.
+   *
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
+   */
+  protected $themeManager;
+
+  /**
+   * Constructs the TwigTweakExtension object.
+   */
+  public function __construct(ModuleHandlerInterface $module_handler, ThemeManagerInterface $theme_manager) {
+    $this->moduleHandler = $module_handler;
+    $this->themeManager = $theme_manager;
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -42,7 +66,7 @@ class TwigTweakExtension extends AbstractExtension {
     $context_options = ['needs_context' => TRUE];
     $all_options = ['needs_environment' => TRUE, 'needs_context' => TRUE];
 
-    return [
+    $functions = [
       new TwigFunction('drupal_view', 'views_embed_view'),
       new TwigFunction('drupal_view_result', 'views_get_view_result'),
       new TwigFunction('drupal_block', [self::class, 'drupalBlock']),
@@ -67,6 +91,11 @@ class TwigTweakExtension extends AbstractExtension {
       new TwigFunction('drupal_breakpoint', [self::class, 'drupalBreakpoint'], $all_options),
       new TwigFunction('drupal_contextual_links', [self::class, 'drupalContextualLinks']),
     ];
+
+    $this->moduleHandler->alter('twig_tweak_functions', $functions);
+    $this->themeManager->alter('twig_tweak_functions', $functions);
+
+    return $functions;
   }
 
   /**
@@ -92,9 +121,24 @@ class TwigTweakExtension extends AbstractExtension {
       $filters[] = new TwigFilter('php', [self::class, 'phpFilter']);
     }
 
+    $this->moduleHandler->alter('twig_tweak_filters', $filters);
+    $this->themeManager->alter('twig_tweak_filters', $filters);
+
     return $filters;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getTests(): array {
+    $tests = [];
+
+    $this->moduleHandler->alter('twig_tweak_tests', $tests);
+    $this->themeManager->alter('twig_tweak_tests', $tests);
+
+    return $tests;
+  }
+
   /**
    * Builds the render array for a block.
    */
diff --git a/tests/src/Functional/TwigTweakTest.php b/tests/src/Functional/TwigTweakTest.php
index 264bdb4..042a8e7 100644
--- a/tests/src/Functional/TwigTweakTest.php
+++ b/tests/src/Functional/TwigTweakTest.php
@@ -358,6 +358,18 @@ final class TwigTweakTest extends BrowserTestBase {
     // -- File URL from media field.
     $xpath = '//div[@class = "tt-file-url-from-media-field" and contains(text(), "/files/image-1.png")]';
     $this->assertXpath($xpath);
+
+    // -- Hook twig_tweak_functions_alter().
+    $xpath = '//div[@class = "tt-functions_alter" and text() = "-=bar=-"]';
+    $this->assertXpath($xpath);
+
+    // -- Hook twig_tweak_filters_alter().
+    $xpath = '//div[@class = "tt-filters_alter" and text() = "bar"]';
+    $this->assertXpath($xpath);
+
+    // -- Hook twig_tweak_tests_alter().
+    $xpath = '//div[@class = "tt-tests_alter" and text() = "Yes"]';
+    $this->assertXpath($xpath);
   }
 
   /**
diff --git a/tests/twig_tweak_test/templates/twig-tweak-test.html.twig b/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
index dcc9f28..0cca8eb 100644
--- a/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
+++ b/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
@@ -89,4 +89,7 @@
   <div class="tt-file-url-from-image-field">{{ node.field_image|file_url }}</div>
   <div class="tt-file-url-from-image-field-delta">{{ node.field_image[0]|file_url }}</div>
   <div class="tt-file-url-from-media-field">{{ node.field_media|file_url }}</div>
+  <div class="tt-functions_alter">{{ foo('bar') }}</div>
+  <div class="tt-filters_alter">{{ 'foo'|bar }}</div>
+  <div class="tt-tests_alter">{{ 'ok' is ok ? 'Yes' : 'No' }}</div>
 </div>
diff --git a/tests/twig_tweak_test/twig_tweak_test.module b/tests/twig_tweak_test/twig_tweak_test.module
index 02c9795..6114e8d 100644
--- a/tests/twig_tweak_test/twig_tweak_test.module
+++ b/tests/twig_tweak_test/twig_tweak_test.module
@@ -11,6 +11,9 @@ use Drupal\Core\Access\AccessResultInterface;
 use Drupal\file\FileInterface;
 use Drupal\node\Entity\Node;
 use Drupal\node\NodeInterface;
+use Twig\TwigFilter;
+use Twig\TwigFunction;
+use Twig\TwigTest;
 
 /**
  * Implements hook_page_bottom().
@@ -72,3 +75,30 @@ function twig_tweak_test_file_access(FileInterface $file): AccessResultInterface
   $result->setCacheMaxAge(70);
   return $result;
 }
+
+/**
+ * Implements hook_twig_tweak_functions_alter().
+ */
+function twig_tweak_test_twig_tweak_functions_alter(array &$functions) {
+  $functions[] = new TwigFunction('foo', function (string $value): string {
+    return "-=$value=-";
+  });
+}
+
+/**
+ * Implements hook_twig_tweak_filters_alter().
+ */
+function twig_tweak_test_twig_tweak_filters_alter(array &$filters) {
+  $filters[] = new TwigFilter('bar', function (string $input): string {
+    return str_replace('foo', 'bar', $input);
+  });
+}
+
+/**
+ * Implements hook_twig_tweak_tests_alter().
+ */
+function twig_tweak_test_twig_tweak_tests_alter(array &$tests) {
+  $tests[] = new TwigTest('ok', function ($input): bool {
+    return $input == 'ok';
+  });
+}
diff --git a/twig_tweak.api.php b/twig_tweak.api.php
new file mode 100644
index 0000000..31d0bfa
--- /dev/null
+++ b/twig_tweak.api.php
@@ -0,0 +1,67 @@
+<?php
+
+/**
+ * @file
+ * Hooks specific to the Twig Tweak module.
+ */
+
+use Drupal\Component\Utility\Unicode;
+use Drupal\node\NodeInterface;
+use Twig\TwigFunction;
+use Twig\TwigTest;
+
+/**
+ * @addtogroup hooks
+ * @{
+ */
+
+/**
+ * Alters Twig Tweak functions.
+ *
+ * @param \Twig\TwigFunction[] $functions
+ *   Twig functions to alter.
+ */
+function hook_twig_tweak_functions_alter(array &$functions): void {
+  // A simple way to implement lazy loaded global variables.
+  $functions[] = new TwigFunction('var', function (string $name) {
+    $value = NULL;
+    switch ($name) {
+      case 'foo':
+        $value = 'Foo';
+        break;
+
+      case 'bar':
+        $value = 'Bar';
+        break;
+    }
+    return $value;
+  });
+}
+
+/**
+ * Alters Twig Tweak filters.
+ *
+ * @param \Twig\TwigFilter[] $filters
+ *   Twig filters to alter.
+ */
+function hook_twig_tweak_filters_alter(array &$filters): void {
+  $filters[] = new TwigFunction('str_pad', 'str_pad');
+  $filters[] = new TwigFunction('ucfirst', [Unicode::class, 'ucfirst']);
+  $filters[] = new TwigFunction('lcfirst', [Unicode::class, 'lcfirst']);
+}
+
+/**
+ * Alters Twig Tweak tests.
+ *
+ * @param \Twig\TwigTest[] $tests
+ *   Twig tests to alter.
+ */
+function hook_twig_tweak_tests_alter(array &$tests): void {
+  $tests[] = new TwigTest('outdated', function (NodeInterface $node): bool {
+    return \Drupal::time()->getRequestTime() - $node->getCreatedTime() > 3600 * 24 * 365;
+  });
+}
+
+/**
+ * @} End of "addtogroup hooks".
+ */
diff --git a/twig_tweak.services.yml b/twig_tweak.services.yml
index 70d11cf..8ae8efa 100644
--- a/twig_tweak.services.yml
+++ b/twig_tweak.services.yml
@@ -1,6 +1,7 @@
 services:
   twig_tweak.twig_extension:
     class: Drupal\twig_tweak\TwigTweakExtension
+    arguments: ['@module_handler', '@theme.manager']
     tags:
       - { name: twig.extension }