diff --git a/config/audit.php b/config/audit.php
index 612455db..812a4837 100644
--- a/config/audit.php
+++ b/config/audit.php
@@ -30,14 +30,18 @@
     | User Keys, Model
     |--------------------------------------------------------------------------
     |
-    | Define the User primary key, foreign key and Eloquent model.
+    | Define the User Eloquent model, morph prefix and authentication guards
+    | to use in the User resolver.
     |
     */
 
     'user' => [
-        'primary_key' => 'id',
-        'foreign_key' => 'user_id',
-        'model'       => App\User::class,
+        'model'        => App\User::class,
+        'morph_prefix' => 'user',
+        'guards'       => [
+            'web',
+            'api',
+        ],
     ],
 
     /*
diff --git a/database/migrations/audits.stub b/database/migrations/audits.stub
index d9f8bf3f..d380416e 100644
--- a/database/migrations/audits.stub
+++ b/database/migrations/audits.stub
@@ -27,7 +27,7 @@ class CreateAuditsTable extends Migration
     {
         Schema::create('audits', function (Blueprint $table) {
             $table->increments('id');
-            $table->unsignedInteger('user_id')->nullable();
+            $table->nullableMorphs('user');
             $table->string('event');
             $table->morphs('auditable');
             $table->text('old_values')->nullable();
diff --git a/src/Audit.php b/src/Audit.php
index 9c7ab265..8484a9bf 100644
--- a/src/Audit.php
+++ b/src/Audit.php
@@ -16,7 +16,6 @@
 
 use DateTimeInterface;
 use Illuminate\Database\Eloquent\Model;
-use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\MorphTo;
 use Illuminate\Support\Facades\Config;
 
@@ -70,13 +69,9 @@ public function auditable(): MorphTo
     /**
      * {@inheritdoc}
      */
-    public function user(): BelongsTo
+    public function user(): MorphTo
     {
-        return $this->belongsTo(
-            Config::get('audit.user.model'),
-            Config::get('audit.user.foreign_key', 'user_id'),
-            Config::get('audit.user.primary_key', 'id')
-        );
+        return $this->morphTo();
     }
 
     /**
@@ -84,6 +79,8 @@ public function user(): BelongsTo
      */
     public function resolveData(): array
     {
+        $morphPrefix = Config::get('audit.user.morph_prefix', 'user');
+
         // Metadata
         $this->data = [
             'audit_id'         => $this->id,
@@ -94,7 +91,8 @@ public function resolveData(): array
             'audit_tags'       => $this->tags,
             'audit_created_at' => $this->serializeDate($this->created_at),
             'audit_updated_at' => $this->serializeDate($this->updated_at),
-            'user_id'          => $this->getAttribute(Config::get('audit.user.foreign_key', 'user_id')),
+            'user_id'          => $this->getAttribute($morphPrefix.'_id'),
+            'user_type'        => $this->getAttribute($morphPrefix.'_type'),
         ];
 
         if ($this->user) {
diff --git a/src/Auditable.php b/src/Auditable.php
index a360f9ca..afff8572 100644
--- a/src/Auditable.php
+++ b/src/Auditable.php
@@ -278,21 +278,24 @@ public function toAudit(): array
             }
         }
 
-        $userForeignKey = Config::get('audit.user.foreign_key', 'user_id');
+        $morphPrefix = Config::get('audit.user.morph_prefix', 'user');
 
         $tags = implode(',', $this->generateTags());
 
+        $user = $this->resolveUser();
+
         return $this->transformAudit([
-            'old_values'     => $old,
-            'new_values'     => $new,
-            'event'          => $this->auditEvent,
-            'auditable_id'   => $this->getKey(),
-            'auditable_type' => $this->getMorphClass(),
-            $userForeignKey  => $this->resolveUser(),
-            'url'            => $this->resolveUrl(),
-            'ip_address'     => $this->resolveIpAddress(),
-            'user_agent'     => $this->resolveUserAgent(),
-            'tags'           => empty($tags) ? null : $tags,
+            'old_values'         => $old,
+            'new_values'         => $new,
+            'event'              => $this->auditEvent,
+            'auditable_id'       => $this->getKey(),
+            'auditable_type'     => $this->getMorphClass(),
+            $morphPrefix.'_id'   => $user ? $user->getAuthIdentifier() : null,
+            $morphPrefix.'_type' => $user ? $user->getMorphClass() : null,
+            'url'                => $this->resolveUrl(),
+            'ip_address'         => $this->resolveIpAddress(),
+            'user_agent'         => $this->resolveUserAgent(),
+            'tags'               => empty($tags) ? null : $tags,
         ]);
     }
 
diff --git a/src/Contracts/Audit.php b/src/Contracts/Audit.php
index 04699d6c..427387cb 100644
--- a/src/Contracts/Audit.php
+++ b/src/Contracts/Audit.php
@@ -14,7 +14,6 @@
 
 namespace OwenIt\Auditing\Contracts;
 
-use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\MorphTo;
 
 interface Audit
@@ -43,9 +42,9 @@ public function auditable(): MorphTo;
     /**
      * User responsible for the changes.
      *
-     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
+     * @return \Illuminate\Database\Eloquent\Relations\MorphTo
      */
-    public function user(): BelongsTo;
+    public function user(): MorphTo;
 
     /**
      * Audit data resolver.
diff --git a/src/Resolvers/UserResolver.php b/src/Resolvers/UserResolver.php
index b5c9c61d..8e06e969 100644
--- a/src/Resolvers/UserResolver.php
+++ b/src/Resolvers/UserResolver.php
@@ -15,6 +15,7 @@
 namespace OwenIt\Auditing\Resolvers;
 
 use Illuminate\Support\Facades\Auth;
+use Illuminate\Support\Facades\Config;
 
 class UserResolver implements \OwenIt\Auditing\Contracts\UserResolver
 {
@@ -23,6 +24,15 @@ class UserResolver implements \OwenIt\Auditing\Contracts\UserResolver
      */
     public static function resolve()
     {
-        return Auth::check() ? Auth::user()->getAuthIdentifier() : null;
+        $guards = Config::get('audit.user.guards', [
+            'web',
+            'api',
+        ]);
+
+        foreach ($guards as $guard) {
+            if (Auth::guard($guard)->check()) {
+                return Auth::guard($guard)->user();
+            }
+        }
     }
 }
diff --git a/tests/Unit/AuditTest.php b/tests/Unit/AuditTest.php
index 49c06bbe..b60132fe 100644
--- a/tests/Unit/AuditTest.php
+++ b/tests/Unit/AuditTest.php
@@ -16,41 +16,12 @@
 
 use Carbon\Carbon;
 use DateTimeInterface;
-use Mockery;
 use OwenIt\Auditing\Models\Audit;
 use OwenIt\Auditing\Tests\Models\Article;
 use OwenIt\Auditing\Tests\Models\User;
 
 class AuditTest extends AuditingTestCase
 {
-    /**
-     * @group Audit::user
-     * @test
-     */
-    public function itRelatesToUserWithCustomKeys()
-    {
-        $audit = Mockery::mock(Audit::class)
-            ->makePartial();
-
-        $this->app['config']->set('audit.user.model', User::class);
-        $this->app['config']->set('audit.user.primary_key', 'pk_id');
-        $this->app['config']->set('audit.user.foreign_key', 'fk_id');
-
-        $this->assertInstanceOf(User::class, $audit->user()->getRelated());
-
-        // Up to Laravel 5.3, the ownerKey attribute was called otherKey
-        if (method_exists($audit->user(), 'getOtherKey')) {
-            $this->assertSame('pk_id', $audit->user()->getOtherKey());
-        }
-
-        // From Laravel 5.4 onward, the otherKey attribute was renamed to ownerKey
-        if (method_exists($audit->user(), 'getOwnerKey')) {
-            $this->assertSame('pk_id', $audit->user()->getOwnerKey());
-        }
-
-        $this->assertSame('fk_id', $audit->user()->getForeignKey());
-    }
-
     /**
      * @group Audit::resolveData
      * @test
@@ -68,7 +39,7 @@ public function itResolvesAuditData()
 
         $audit = $article->audits()->first();
 
-        $this->assertCount(14, $resolvedData = $audit->resolveData());
+        $this->assertCount(15, $resolvedData = $audit->resolveData());
 
         $this->assertArraySubset([
             'audit_id'         => 1,
@@ -80,6 +51,7 @@ public function itResolvesAuditData()
             'audit_created_at' => $audit->created_at->toDateTimeString(),
             'audit_updated_at' => $audit->updated_at->toDateTimeString(),
             'user_id'          => null,
+            'user_type'        => null,
             'new_title'        => 'How To Audit Eloquent Models',
             'new_content'      => 'First step: install the laravel-auditing package.',
             'new_published_at' => $now->toDateTimeString(),
@@ -114,7 +86,7 @@ public function itResolvesAuditDataIncludingUserAttributes()
 
         $audit = $article->audits()->first();
 
-        $this->assertCount(20, $resolvedData = $audit->resolveData());
+        $this->assertCount(21, $resolvedData = $audit->resolveData());
 
         $this->assertArraySubset([
             'audit_id'         => 2,
@@ -126,6 +98,7 @@ public function itResolvesAuditDataIncludingUserAttributes()
             'audit_created_at' => $audit->created_at->toDateTimeString(),
             'audit_updated_at' => $audit->updated_at->toDateTimeString(),
             'user_id'          => '1',
+            'user_type'        => User::class,
             'user_is_admin'    => '1',
             'user_first_name'  => 'rick',
             'user_last_name'   => 'Sanchez',
@@ -164,7 +137,7 @@ public function itReturnsTheAppropriateAuditableDataValues()
         ])->audits()->first();
 
         // Resolve data, making it available to the getDataValue() method
-        $this->assertCount(20, $audit->resolveData());
+        $this->assertCount(21, $audit->resolveData());
 
         // Mutate value
         $this->assertSame('HOW TO AUDIT ELOQUENT MODELS', $audit->getDataValue('new_title'));
@@ -194,7 +167,7 @@ public function itReturnsAuditMetadataAsArray()
     {
         $audit = factory(Article::class)->create()->audits()->first();
 
-        $this->assertCount(9, $metadata = $audit->getMetadata());
+        $this->assertCount(10, $metadata = $audit->getMetadata());
 
         $this->assertArraySubset([
             'audit_id'         => 1,
@@ -206,6 +179,7 @@ public function itReturnsAuditMetadataAsArray()
             'audit_created_at' => $audit->created_at->toDateTimeString(),
             'audit_updated_at' => $audit->updated_at->toDateTimeString(),
             'user_id'          => null,
+            'user_type'        => null,
         ], $metadata, true);
     }
 
@@ -226,7 +200,7 @@ public function itReturnsAuditMetadataIncludingUserAttributesAsArray()
 
         $audit = factory(Article::class)->create()->audits()->first();
 
-        $this->assertCount(15, $metadata = $audit->getMetadata());
+        $this->assertCount(16, $metadata = $audit->getMetadata());
 
         $this->assertArraySubset([
             'audit_id'         => 2,
@@ -238,6 +212,7 @@ public function itReturnsAuditMetadataIncludingUserAttributesAsArray()
             'audit_created_at' => $audit->created_at->toDateTimeString(),
             'audit_updated_at' => $audit->updated_at->toDateTimeString(),
             'user_id'          => 1,
+            'user_type'        => User::class,
             'user_is_admin'    => true,
             'user_first_name'  => 'Rick',
             'user_last_name'   => 'Sanchez',
@@ -267,7 +242,8 @@ public function itReturnsAuditMetadataAsJsonString()
     "audit_tags": null,
     "audit_created_at": "$audit->created_at",
     "audit_updated_at": "$audit->updated_at",
-    "user_id": null
+    "user_id": null,
+    "user_type": null
 }
 EOF;
 
@@ -304,6 +280,7 @@ public function itReturnsAuditMetadataIncludingUserAttributesAsJsonString()
     "audit_created_at": "$audit->created_at",
     "audit_updated_at": "$audit->updated_at",
     "user_id": 1,
+    "user_type": "OwenIt\\\Auditing\\\Tests\\\Models\\\User",
     "user_is_admin": true,
     "user_first_name": "Rick",
     "user_last_name": "Sanchez",
diff --git a/tests/Unit/AuditableObserverTest.php b/tests/Unit/AuditableObserverTest.php
index fca6deb8..7f631d55 100644
--- a/tests/Unit/AuditableObserverTest.php
+++ b/tests/Unit/AuditableObserverTest.php
@@ -41,7 +41,7 @@ public function itExecutesTheAuditorSuccessfully(string $eventMethod, bool $expe
 
         $this->assertSame($expectedBefore, $observer::$restoring);
 
-        call_user_func([$observer, $eventMethod], $model);
+        $observer->$eventMethod($model);
 
         $this->assertSame($expectedAfter, $observer::$restoring);
     }
@@ -49,7 +49,7 @@ public function itExecutesTheAuditorSuccessfully(string $eventMethod, bool $expe
     /**
      * @return array
      */
-    public function auditableObserverTestProvider()
+    public function auditableObserverTestProvider(): array
     {
         return [
             [
diff --git a/tests/Unit/AuditableTest.php b/tests/Unit/AuditableTest.php
index b9488fff..d81c55f5 100644
--- a/tests/Unit/AuditableTest.php
+++ b/tests/Unit/AuditableTest.php
@@ -260,7 +260,7 @@ public function itFailsWhenTheCustomAttributeGettersAreMissing(
     /**
      * @return array
      */
-    public function auditCustomAttributeGetterFailTestProvider()
+    public function auditCustomAttributeGetterFailTestProvider(): array
     {
         return [
             [
@@ -388,7 +388,7 @@ public function itReturnsTheAuditData()
 
         $model->setAuditEvent('created');
 
-        $this->assertCount(10, $auditData = $model->toAudit());
+        $this->assertCount(11, $auditData = $model->toAudit());
 
         $this->assertArraySubset([
             'old_values' => [],
@@ -402,6 +402,7 @@ public function itReturnsTheAuditData()
             'auditable_id'   => null,
             'auditable_type' => Article::class,
             'user_id'        => null,
+            'user_type'      => null,
             'url'            => 'console',
             'ip_address'     => '127.0.0.1',
             'user_agent'     => 'Symfony/3.X',
@@ -413,12 +414,27 @@ public function itReturnsTheAuditData()
      * @group Auditable::setAuditEvent
      * @group Auditable::toAudit
      * @test
+     *
+     * @dataProvider userResolverProvider
+     *
+     * @param string $guard
+     * @param string $driver
+     * @param int    $id
+     * @param string $type
      */
-    public function itReturnsTheAuditDataIncludingUserAttributes()
-    {
+    public function itReturnsTheAuditDataIncludingUserAttributes(
+        string $guard,
+        string $driver,
+        int $id = null,
+        string $type = null
+    ) {
+        $this->app['config']->set('audit.user.guards', [
+            $guard,
+        ]);
+
         $user = factory(User::class)->create();
 
-        $this->actingAs($user);
+        $this->actingAs($user, $driver);
 
         $now = Carbon::now();
 
@@ -431,7 +447,7 @@ public function itReturnsTheAuditDataIncludingUserAttributes()
 
         $model->setAuditEvent('created');
 
-        $this->assertCount(10, $auditData = $model->toAudit());
+        $this->assertCount(11, $auditData = $model->toAudit());
 
         $this->assertArraySubset([
             'old_values' => [],
@@ -444,7 +460,8 @@ public function itReturnsTheAuditDataIncludingUserAttributes()
             'event'          => 'created',
             'auditable_id'   => null,
             'auditable_type' => Article::class,
-            'user_id'        => 1,
+            'user_id'        => $id,
+            'user_type'      => $type,
             'url'            => 'console',
             'ip_address'     => '127.0.0.1',
             'user_agent'     => 'Symfony/3.X',
@@ -452,6 +469,39 @@ public function itReturnsTheAuditDataIncludingUserAttributes()
         ], $auditData, true);
     }
 
+    /**
+     * @return array
+     */
+    public function userResolverProvider(): array
+    {
+        return [
+            [
+                'api',
+                'web',
+                null,
+                null,
+            ],
+            [
+                'web',
+                'api',
+                null,
+                null,
+            ],
+            [
+                'api',
+                'api',
+                1,
+                User::class,
+            ],
+            [
+                'web',
+                'web',
+                1,
+                User::class,
+            ],
+        ];
+    }
+
     /**
      * @group Auditable::setAuditEvent
      * @group Auditable::toAudit
@@ -479,7 +529,7 @@ public function itExcludesAttributesFromTheAuditDataWhenInStrictMode()
 
         $model->setAuditEvent('created');
 
-        $this->assertCount(10, $auditData = $model->toAudit());
+        $this->assertCount(11, $auditData = $model->toAudit());
 
         $this->assertArraySubset([
             'old_values' => [],
@@ -491,6 +541,7 @@ public function itExcludesAttributesFromTheAuditDataWhenInStrictMode()
             'auditable_id'   => null,
             'auditable_type' => Article::class,
             'user_id'        => null,
+            'user_type'      => null,
             'url'            => 'console',
             'ip_address'     => '127.0.0.1',
             'user_agent'     => 'Symfony/3.X',
@@ -587,7 +638,7 @@ public function transformAudit(array $data): array
 
         $model->setAuditEvent('created');
 
-        $this->assertCount(10, $auditData = $model->toAudit());
+        $this->assertCount(11, $auditData = $model->toAudit());
 
         $this->assertArraySubset([
             'new_values' => [
@@ -1004,7 +1055,7 @@ public function itTransitionsToAnotherModelState(
     /**
      * @return array
      */
-    public function auditableTransitionTestProvider()
+    public function auditableTransitionTestProvider(): array
     {
         return [
             //
diff --git a/tests/database/factories/AuditFactory.php b/tests/database/factories/AuditFactory.php
index 31ec886e..f9f017f3 100644
--- a/tests/database/factories/AuditFactory.php
+++ b/tests/database/factories/AuditFactory.php
@@ -28,6 +28,7 @@
         'user_id' => function () {
             return factory(User::class)->create()->id;
         },
+        'user_type'    => User::class,
         'event'        => 'updated',
         'auditable_id' => function () {
             return factory(Article::class)->create()->id;
diff --git a/tests/database/migrations/0000_00_00_000001_create_audits_test_table.php b/tests/database/migrations/0000_00_00_000001_create_audits_test_table.php
index 42c16426..20afbc3b 100644
--- a/tests/database/migrations/0000_00_00_000001_create_audits_test_table.php
+++ b/tests/database/migrations/0000_00_00_000001_create_audits_test_table.php
@@ -26,7 +26,7 @@ public function up()
     {
         Schema::create('audits', function (Blueprint $table) {
             $table->increments('id');
-            $table->unsignedInteger('user_id')->nullable();
+            $table->nullableMorphs('user');
             $table->string('event');
             $table->morphs('auditable');
             $table->text('old_values')->nullable();