diff --git a/js/src/admin/components/ExtensionsPage.js b/js/src/admin/components/ExtensionsPage.js
index 5a50fea6ea..0ef4e9f9c4 100644
--- a/js/src/admin/components/ExtensionsPage.js
+++ b/js/src/admin/components/ExtensionsPage.js
@@ -123,6 +123,7 @@ export default class ExtensionsPage extends Page {
         url: app.forum.attribute('apiUrl') + '/extensions/' + id,
         method: 'PATCH',
         body: { enabled: !enabled },
+        errorHandler: this.onerror.bind(this),
       })
       .then(() => {
         if (!enabled) localStorage.setItem('enabledExtension', id);
@@ -131,4 +132,23 @@ export default class ExtensionsPage extends Page {
 
     app.modal.show(LoadingModal);
   }
+
+  onerror(e) {
+    // We need to give the modal animation time to start; if we close the modal too early,
+    // it breaks the bootstrap modal library.
+    // TODO: This workaround should be removed when we move away from bootstrap JS for modals.
+    setTimeout(() => {
+      app.modal.close();
+
+      const error = JSON.parse(e.responseText).errors[0];
+
+      app.alerts.show(
+        { type: 'error' },
+        app.translator.trans(`core.lib.error.${error.code}_message`, {
+          extension: error.extension,
+          extensions: error.extensions.join(', '),
+        })
+      );
+    }, 250);
+  }
 }
diff --git a/src/Extension/Exception/DependentExtensionsException.php b/src/Extension/Exception/DependentExtensionsException.php
new file mode 100644
index 0000000000..ab26347061
--- /dev/null
+++ b/src/Extension/Exception/DependentExtensionsException.php
@@ -0,0 +1,47 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extension\Exception;
+
+use Exception;
+use Flarum\Extension\Extension;
+
+/**
+ * This exception is thrown when someone attempts to disable an extension
+ * that other enabled extensions depend on.
+ */
+class DependentExtensionsException extends Exception
+{
+    public $extension;
+    public $dependent_extensions;
+
+    /**
+     * @param $extension: The extension we are attempting to disable.
+     * @param $dependent_extensions: Enabled Flarum extensions that depend on this extension.
+     */
+    public function __construct(Extension $extension, array $dependent_extensions)
+    {
+        $this->extension = $extension;
+        $this->dependent_extensions = $dependent_extensions;
+
+        parent::__construct($extension->getId().' could not be disabled, because it is a dependency of: '.implode(', ', $this->getDependentExtensionIds()));
+    }
+
+    /**
+     * Get array of IDs for extensions that depend on this extension.
+     *
+     * @return array
+     */
+    public function getDependentExtensionIds()
+    {
+        return array_map(function (Extension $extension) {
+            return $extension->getId();
+        }, $this->dependent_extensions);
+    }
+}
diff --git a/src/Extension/Exception/DependentExtensionsExceptionHandler.php b/src/Extension/Exception/DependentExtensionsExceptionHandler.php
new file mode 100644
index 0000000000..a8ad877274
--- /dev/null
+++ b/src/Extension/Exception/DependentExtensionsExceptionHandler.php
@@ -0,0 +1,34 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extension\Exception;
+
+use Flarum\Foundation\ErrorHandling\HandledError;
+
+class DependentExtensionsExceptionHandler
+{
+    public function handle(DependentExtensionsException $e): HandledError
+    {
+        return (new HandledError(
+            $e,
+            'dependent_extensions',
+            409
+        ))->withDetails($this->errorDetails($e));
+    }
+
+    protected function errorDetails(DependentExtensionsException $e): array
+    {
+        return [
+            [
+                'extension' => $e->extension->getId(),
+                'extensions' => $e->getDependentExtensionIds(),
+            ]
+        ];
+    }
+}
diff --git a/src/Extension/Exception/MissingDependenciesException.php b/src/Extension/Exception/MissingDependenciesException.php
new file mode 100644
index 0000000000..114348ca40
--- /dev/null
+++ b/src/Extension/Exception/MissingDependenciesException.php
@@ -0,0 +1,47 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extension\Exception;
+
+use Exception;
+use Flarum\Extension\Extension;
+
+/**
+ * This exception is thrown when someone attempts to enable an extension
+ * whose Flarum extension dependencies are not all enabled.
+ */
+class MissingDependenciesException extends Exception
+{
+    public $extension;
+    public $missing_dependencies;
+
+    /**
+     * @param $extension: The extension we are attempting to enable.
+     * @param $missing_dependencies: Extensions that this extension depends on, and are not enabled.
+     */
+    public function __construct(Extension $extension, array $missing_dependencies = null)
+    {
+        $this->extension = $extension;
+        $this->missing_dependencies = $missing_dependencies;
+
+        parent::__construct($extension->getId().' could not be enabled, because it depends on: '.implode(', ', $this->getMissingDependencyIds()));
+    }
+
+    /**
+     * Get array of IDs for missing (disabled) extensions that this extension depends on.
+     *
+     * @return array
+     */
+    public function getMissingDependencyIds()
+    {
+        return array_map(function (Extension $extension) {
+            return $extension->getId();
+        }, $this->missing_dependencies);
+    }
+}
diff --git a/src/Extension/Exception/MissingDependenciesExceptionHandler.php b/src/Extension/Exception/MissingDependenciesExceptionHandler.php
new file mode 100644
index 0000000000..39f8980cc2
--- /dev/null
+++ b/src/Extension/Exception/MissingDependenciesExceptionHandler.php
@@ -0,0 +1,34 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extension\Exception;
+
+use Flarum\Foundation\ErrorHandling\HandledError;
+
+class MissingDependenciesExceptionHandler
+{
+    public function handle(MissingDependenciesException $e): HandledError
+    {
+        return (new HandledError(
+            $e,
+            'missing_dependencies',
+            409
+        ))->withDetails($this->errorDetails($e));
+    }
+
+    protected function errorDetails(MissingDependenciesException $e): array
+    {
+        return [
+            [
+                'extension' => $e->extension->getId(),
+                'extensions' => $e->getMissingDependencyIds(),
+            ]
+        ];
+    }
+}
diff --git a/src/Extension/Extension.php b/src/Extension/Extension.php
index 3707bfae08..ea7b90ed04 100644
--- a/src/Extension/Extension.php
+++ b/src/Extension/Extension.php
@@ -15,6 +15,7 @@
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Contracts\Support\Arrayable;
 use Illuminate\Support\Arr;
+use Illuminate\Support\Collection;
 use Illuminate\Support\Str;
 use League\Flysystem\Adapter\Local;
 use League\Flysystem\Filesystem;
@@ -51,6 +52,14 @@ class Extension implements Arrayable
         'jpg' => 'image/jpeg',
     ];
 
+    protected static function nameToId($name)
+    {
+        list($vendor, $package) = explode('/', $name);
+        $package = str_replace(['flarum-ext-', 'flarum-'], '', $package);
+
+        return "$vendor-$package";
+    }
+
     /**
      * Unique Id of the extension.
      *
@@ -60,6 +69,7 @@ class Extension implements Arrayable
      * @var string
      */
     protected $id;
+
     /**
      * The directory of this extension.
      *
@@ -74,6 +84,13 @@ class Extension implements Arrayable
      */
     protected $composerJson;
 
+    /**
+     * The IDs of all Flarum extensions that this extension depends on.
+     *
+     * @var string[]
+     */
+    protected $extensionDependencyIds;
+
     /**
      * Whether the extension is installed.
      *
@@ -104,9 +121,7 @@ public function __construct($path, $composerJson)
      */
     protected function assignId()
     {
-        list($vendor, $package) = explode('/', $this->name);
-        $package = str_replace(['flarum-ext-', 'flarum-'], '', $package);
-        $this->id = "$vendor-$package";
+        $this->id = static::nameToId($this->name);
     }
 
     public function extend(Container $app)
@@ -182,6 +197,24 @@ public function setVersion($version)
         return $this;
     }
 
+    /**
+     * Get the list of flarum extensions that this extension depends on.
+     *
+     * @param array $extensionSet: An associative array where keys are the composer package names
+     *                             of installed extensions. Used to figure out which dependencies
+     *                             are flarum extensions.
+     */
+    public function calculateDependencies($extensionSet)
+    {
+        $this->extensionDependencyIds = (new Collection(Arr::get($this->composerJson, 'require', [])))
+            ->keys()
+            ->filter(function ($key) use ($extensionSet) {
+                return array_key_exists($key, $extensionSet);
+            })->map(function ($key) {
+                return static::nameToId($key);
+            })->toArray();
+    }
+
     /**
      * @return string
      */
@@ -253,6 +286,16 @@ public function getPath()
         return $this->path;
     }
 
+    /**
+     * The IDs of all Flarum extensions that this extension depends on.
+     *
+     * @return array
+     */
+    public function getExtensionDependencyIds()
+    {
+        return $this->extensionDependencyIds;
+    }
+
     private function getExtenders(): array
     {
         $extenderFile = $this->getExtenderFile();
@@ -363,12 +406,13 @@ public function migrate(Migrator $migrator, $direction = 'up')
     public function toArray()
     {
         return (array) array_merge([
-            'id'            => $this->getId(),
-            'version'       => $this->getVersion(),
-            'path'          => $this->path,
-            'icon'          => $this->getIcon(),
-            'hasAssets'     => $this->hasAssets(),
-            'hasMigrations' => $this->hasMigrations(),
+            'id'                     => $this->getId(),
+            'version'                => $this->getVersion(),
+            'path'                   => $this->getPath(),
+            'icon'                   => $this->getIcon(),
+            'hasAssets'              => $this->hasAssets(),
+            'hasMigrations'          => $this->hasMigrations(),
+            'extensionDependencyIds' => $this->getExtensionDependencyIds(),
         ], $this->composerJson);
     }
 }
diff --git a/src/Extension/ExtensionManager.php b/src/Extension/ExtensionManager.php
index 40136e3484..7b393dc5aa 100644
--- a/src/Extension/ExtensionManager.php
+++ b/src/Extension/ExtensionManager.php
@@ -83,11 +83,18 @@ public function getExtensions()
             // Composer 2.0 changes the structure of the installed.json manifest
             $installed = $installed['packages'] ?? $installed;
 
+            // We calculate and store a set of composer package names for all installed Flarum extensions,
+            // so we know what is and isn't a flarum extension in `calculateDependencies`.
+            // Using keys of an associative array allows us to do these checks in constant time.
+            $installedSet = [];
+
             foreach ($installed as $package) {
                 if (Arr::get($package, 'type') != 'flarum-extension' || empty(Arr::get($package, 'name'))) {
                     continue;
                 }
 
+                $installedSet[Arr::get($package, 'name')] = true;
+
                 $path = isset($package['install-path'])
                     ? $this->paths->vendor.'/composer/'.$package['install-path']
                     : $this->paths->vendor.'/'.Arr::get($package, 'name');
@@ -101,6 +108,11 @@ public function getExtensions()
 
                 $extensions->put($extension->getId(), $extension);
             }
+
+            foreach ($extensions as $extension) {
+                $extension->calculateDependencies($installedSet);
+            }
+
             $this->extensions = $extensions->sortBy(function ($extension, $name) {
                 return $extension->composerJsonAttribute('extra.flarum-extension.title');
             });
@@ -133,17 +145,27 @@ public function enable($name)
 
         $extension = $this->getExtension($name);
 
-        $this->dispatcher->dispatch(new Enabling($extension));
+        $missingDependencies = [];
+        $enabledIds = $this->getEnabled();
+        foreach ($extension->getExtensionDependencyIds() as $dependencyId) {
+            if (! in_array($dependencyId, $enabledIds)) {
+                $missingDependencies[] = $this->getExtension($dependencyId);
+            }
+        }
 
-        $enabled = $this->getEnabled();
+        if (! empty($missingDependencies)) {
+            throw new Exception\MissingDependenciesException($extension, $missingDependencies);
+        }
 
-        $enabled[] = $name;
+        $this->dispatcher->dispatch(new Enabling($extension));
+
+        $enabledIds[] = $name;
 
         $this->migrate($extension);
 
         $this->publishAssets($extension);
 
-        $this->setEnabled($enabled);
+        $this->setEnabled($enabledIds);
 
         $extension->enable($this->container);
 
@@ -165,6 +187,18 @@ public function disable($name)
 
         $extension = $this->getExtension($name);
 
+        $dependentExtensions = [];
+
+        foreach ($this->getEnabledExtensions() as $possibleDependent) {
+            if (in_array($extension->getId(), $possibleDependent->getExtensionDependencyIds())) {
+                $dependentExtensions[] = $possibleDependent;
+            }
+        }
+
+        if (! empty($dependentExtensions)) {
+            throw new Exception\DependentExtensionsException($extension, $dependentExtensions);
+        }
+
         $this->dispatcher->dispatch(new Disabling($extension));
 
         unset($enabled[$k]);
diff --git a/src/Foundation/ErrorServiceProvider.php b/src/Foundation/ErrorServiceProvider.php
index 58c827f7fe..02483841de 100644
--- a/src/Foundation/ErrorServiceProvider.php
+++ b/src/Foundation/ErrorServiceProvider.php
@@ -9,6 +9,10 @@
 
 namespace Flarum\Foundation;
 
+use Flarum\Extension\Exception\DependentExtensionsException;
+use Flarum\Extension\Exception\DependentExtensionsExceptionHandler;
+use Flarum\Extension\Exception\MissingDependenciesException;
+use Flarum\Extension\Exception\MissingDependenciesExceptionHandler;
 use Flarum\Foundation\ErrorHandling\ExceptionHandler;
 use Flarum\Foundation\ErrorHandling\LogReporter;
 use Flarum\Foundation\ErrorHandling\Registry;
@@ -57,6 +61,8 @@ public function register()
             return [
                 IlluminateValidationException::class => ExceptionHandler\IlluminateValidationExceptionHandler::class,
                 ValidationException::class => ExceptionHandler\ValidationExceptionHandler::class,
+                DependentExtensionsException::class => DependentExtensionsExceptionHandler::class,
+                MissingDependenciesException::class => MissingDependenciesExceptionHandler::class,
             ];
         });