diff --git a/composer.json b/composer.json
index f454edbca..8b3528a8a 100644
--- a/composer.json
+++ b/composer.json
@@ -44,7 +44,12 @@
     },
     "autoload": {
         "psr-4": { "Zenstruck\\Foundry\\": "src/" },
-        "files": ["src/functions.php", "src/Persistence/functions.php", "src/phpunit_helper.php"]
+        "files": [
+            "src/functions.php",
+            "src/Persistence/functions.php",
+            "src/phpunit_helper.php",
+            "src/InMemory/functions.php"
+        ]
     },
     "autoload-dev": {
         "psr-4": {
diff --git a/src/Configuration.php b/src/Configuration.php
index 0c26525a3..578b0a1ff 100644
--- a/src/Configuration.php
+++ b/src/Configuration.php
@@ -36,11 +36,13 @@ final class Configuration
     /** @var \Closure():self|self|null */
     private static \Closure|self|null $instance = null;
 
+    private bool $inMemory = false;
+
     /**
      * @param InstantiatorCallable $instantiator
      */
     public function __construct(
-        public readonly FactoryRegistry $factories,
+        public readonly FactoryRegistryInterface $factories,
         public readonly Faker\Generator $faker,
         callable $instantiator,
         public readonly StoryRegistry $stories,
@@ -90,4 +92,14 @@ public static function shutdown(): void
         StoryRegistry::reset();
         self::$instance = null;
     }
+
+    public function enableInMemory(): void
+    {
+        $this->inMemory = true;
+    }
+
+    public function isInMemoryEnabled(): bool
+    {
+        return $this->inMemory;
+    }
 }
diff --git a/src/Exception/CannotCreateFactory.php b/src/Exception/CannotCreateFactory.php
new file mode 100644
index 000000000..cca3a8482
--- /dev/null
+++ b/src/Exception/CannotCreateFactory.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\Exception;
+
+final class CannotCreateFactory extends \LogicException
+{
+    public static function argumentCountError(\ArgumentCountError $e): static
+    {
+        return new self('Factories with dependencies (services) cannot be created before foundry is booted.', previous: $e);
+    }
+}
diff --git a/src/Factory.php b/src/Factory.php
index fdf337486..cdc1d8a67 100644
--- a/src/Factory.php
+++ b/src/Factory.php
@@ -12,6 +12,7 @@
 namespace Zenstruck\Foundry;
 
 use Faker;
+use Zenstruck\Foundry\Exception\CannotCreateFactory;
 
 /**
  * @author Kevin Bond <kevinbond@gmail.com>
@@ -33,7 +34,6 @@ public function __construct()
     {
     }
 
-
     /**
      * @param Attributes $attributes
      */
@@ -46,7 +46,7 @@ final public static function new(array|callable $attributes = []): static
         try {
             $factory ??= new static(); // @phpstan-ignore-line
         } catch (\ArgumentCountError $e) {
-            throw new \LogicException('Factories with dependencies (services) cannot be created before foundry is booted.', previous: $e);
+            throw CannotCreateFactory::argumentCountError($e);
         }
 
         return $factory->initialize()->with($attributes);
diff --git a/src/FactoryRegistry.php b/src/FactoryRegistry.php
index 05e96d53c..fc4ee6289 100644
--- a/src/FactoryRegistry.php
+++ b/src/FactoryRegistry.php
@@ -11,12 +11,14 @@
 
 namespace Zenstruck\Foundry;
 
+use Zenstruck\Foundry\Exception\CannotCreateFactory;
+
 /**
  * @author Kevin Bond <kevinbond@gmail.com>
  *
  * @internal
  */
-final class FactoryRegistry
+final class FactoryRegistry implements FactoryRegistryInterface
 {
     /**
      * @param Factory<mixed>[] $factories
@@ -25,14 +27,7 @@ public function __construct(private iterable $factories)
     {
     }
 
-    /**
-     * @template T of Factory
-     *
-     * @param class-string<T> $class
-     *
-     * @return T|null
-     */
-    public function get(string $class): ?Factory
+    public function get(string $class): Factory
     {
         foreach ($this->factories as $factory) {
             if ($class === $factory::class) {
@@ -40,6 +35,10 @@ public function get(string $class): ?Factory
             }
         }
 
-        return null;
+        try {
+            return new $class();
+        } catch (\ArgumentCountError $e) {
+            throw CannotCreateFactory::argumentCountError($e);
+        }
     }
 }
diff --git a/src/FactoryRegistryInterface.php b/src/FactoryRegistryInterface.php
new file mode 100644
index 000000000..f2e5a2026
--- /dev/null
+++ b/src/FactoryRegistryInterface.php
@@ -0,0 +1,29 @@
+<?php
+
+/*
+ * This file is part of the zenstruck/foundry package.
+ *
+ * (c) Kevin Bond <kevinbond@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Zenstruck\Foundry;
+
+/**
+ * @author Nicolas PHILIPPE <nikophil@gmail.com>
+ *
+ * @internal
+ */
+interface FactoryRegistryInterface
+{
+    /**
+     * @template T of Factory
+     *
+     * @param class-string<T> $class
+     *
+     * @return T
+     */
+    public function get(string $class): Factory;
+}
diff --git a/src/InMemory/AsInMemoryRepository.php b/src/InMemory/AsInMemoryRepository.php
new file mode 100644
index 000000000..9559e0bb6
--- /dev/null
+++ b/src/InMemory/AsInMemoryRepository.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\InMemory;
+
+// todo: remove this attribute in favor to interface?
+#[\Attribute(\Attribute::TARGET_CLASS)]
+final class AsInMemoryRepository
+{
+    public function __construct(
+        public readonly string $class
+    )
+    {
+        if (!class_exists($this->class)) {
+            throw new \InvalidArgumentException("Wrong definition for \"AsInMemoryRepository\" attribute: class \"{$this->class}\" does not exist.");
+        }
+    }
+}
diff --git a/src/InMemory/DependencyInjection/InMemoryCompilerPass.php b/src/InMemory/DependencyInjection/InMemoryCompilerPass.php
new file mode 100644
index 000000000..0b96c5d33
--- /dev/null
+++ b/src/InMemory/DependencyInjection/InMemoryCompilerPass.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\InMemory\DependencyInjection;
+
+use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
+use Symfony\Component\DependencyInjection\Compiler\ServiceLocatorTagPass;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Reference;
+use Zenstruck\Foundry\InMemory\InMemoryFactoryRegistry;
+
+/**
+ * @internal
+ */
+final class InMemoryCompilerPass implements CompilerPassInterface
+{
+    public function process(ContainerBuilder $container): void
+    {
+        // create a service locator with all "in memory" repositories, indexed by target class
+        $inMemoryRepositoriesServices = $container->findTaggedServiceIds('foundry.in_memory.repository');
+        $inMemoryRepositoriesLocator = ServiceLocatorTagPass::register(
+            $container,
+            array_combine(
+                array_map(
+                    static function (array $tags) {
+                        if (\count($tags) !== 1) {
+                            throw new \LogicException('Cannot have multiple tags "foundry.in_memory.repository" on a service!');
+                        }
+
+                        return $tags[0]['class'] ?? throw new \LogicException('Invalid tag definition of "foundry.in_memory.repository".');
+                    },
+                    array_values($inMemoryRepositoriesServices)
+                ),
+                array_map(
+                    static fn(string $inMemoryRepositoryId) => new Reference($inMemoryRepositoryId),
+                    array_keys($inMemoryRepositoriesServices)
+                ),
+            )
+        );
+
+        // todo: should we check we only have a 1 repository per class?
+
+        $container->register('.zenstruck_foundry.in_memory.factory_registry')
+            ->setClass(InMemoryFactoryRegistry::class)
+            ->setDecoratedService('.zenstruck_foundry.factory_registry')
+            ->setArgument('$decorated', $container->getDefinition('.zenstruck_foundry.factory_registry'))
+            ->setArgument('$inMemoryRepositories', $inMemoryRepositoriesLocator)
+        ;
+    }
+}
diff --git a/src/InMemory/GenericInMemoryRepository.php b/src/InMemory/GenericInMemoryRepository.php
new file mode 100644
index 000000000..629109e83
--- /dev/null
+++ b/src/InMemory/GenericInMemoryRepository.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\InMemory;
+
+/**
+ * @template T of object
+ * @implements InMemoryRepository<T>
+ *
+ * This class will be used when a specific "in-memory" repository does not exist for a given class.
+ */
+final class GenericInMemoryRepository implements InMemoryRepository
+{
+    /**
+     * @var list<T>
+     */
+    private array $elements = [];
+
+    /**
+     * @param class-string<T> $class
+     */
+    public function __construct(
+        private readonly string $class
+    )
+    {
+    }
+
+    /**
+     * @param T $element
+     */
+    public function _save(object $element): void
+    {
+        if (!$element instanceof $this->class) {
+            throw new \InvalidArgumentException(sprintf('Given object of class "%s" is not an instance of expected "%s"', $element::class, $this->class));
+        }
+
+        if (!in_array($element, $this->elements, true)) {
+            $this->elements[] = $element;
+        }
+    }
+}
diff --git a/src/InMemory/InMemoryFactoryRegistry.php b/src/InMemory/InMemoryFactoryRegistry.php
new file mode 100644
index 000000000..e567a7f4b
--- /dev/null
+++ b/src/InMemory/InMemoryFactoryRegistry.php
@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\InMemory;
+
+use Symfony\Component\DependencyInjection\ServiceLocator;
+use Zenstruck\Foundry\Configuration;
+use Zenstruck\Foundry\Factory;
+use Zenstruck\Foundry\FactoryRegistryInterface;
+use Zenstruck\Foundry\Persistence\PersistentObjectFactory;
+
+/**
+ * @internal
+ * @template T of object
+ */
+final class InMemoryFactoryRegistry implements FactoryRegistryInterface
+{
+    /**
+     * @var array<class-string<T>, GenericInMemoryRepository<T>>
+     */
+    private array $genericInMemoryRepositories = [];
+
+    public function __construct( // @phpstan-ignore-line
+        private readonly FactoryRegistryInterface $decorated,
+        /** @var ServiceLocator<InMemoryRepository> */
+        private readonly ServiceLocator $inMemoryRepositories,
+    ) {
+    }
+
+    /**
+     * @template TFactory of Factory
+     *
+     * @param class-string<TFactory> $class
+     *
+     * @return TFactory
+     */
+    public function get(string $class): Factory
+    {
+        $factory = $this->decorated->get($class);
+
+        if (!$factory instanceof PersistentObjectFactory) { // todo shall we support ObjectFactory as well?
+            return $factory;
+        }
+
+        $configuration = Configuration::instance();
+
+        if (!$configuration->isInMemoryEnabled()) {
+            return $factory;
+        }
+
+        return $factory->withoutPersisting()
+            ->afterInstantiate(
+                fn(object $object) => $this->findInMemoryRepository($factory)->_save($object) // @phpstan-ignore-line
+            );
+    }
+
+    /**
+     * @param PersistentObjectFactory<T> $factory
+     *
+     * @return InMemoryRepository<T>
+     */
+    private function findInMemoryRepository(PersistentObjectFactory $factory): InMemoryRepository
+    {
+        $targetClass = $factory::class();
+        if (!$this->inMemoryRepositories->has($targetClass)) {
+            return $this->genericInMemoryRepositories[$targetClass] ??= new GenericInMemoryRepository($targetClass);
+        }
+
+        return $this->inMemoryRepositories->get($targetClass);
+    }
+}
diff --git a/src/InMemory/InMemoryRepository.php b/src/InMemory/InMemoryRepository.php
new file mode 100644
index 000000000..bfef53880
--- /dev/null
+++ b/src/InMemory/InMemoryRepository.php
@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\InMemory;
+
+/**
+ * @template T of object
+ */
+interface InMemoryRepository
+{
+    /**
+     * @param T $element
+     */
+    public function _save(object $element): void;
+}
diff --git a/src/InMemory/functions.php b/src/InMemory/functions.php
new file mode 100644
index 000000000..1e1714066
--- /dev/null
+++ b/src/InMemory/functions.php
@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of the zenstruck/foundry package.
+ *
+ * (c) Kevin Bond <kevinbond@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Zenstruck\Foundry\InMemory;
+
+use Zenstruck\Foundry\Configuration;
+
+/**
+ * Enable "in memory" repositories globally.
+ */
+function enable_in_memory(): void
+{
+    Configuration::instance()->enableInMemory();
+}
diff --git a/src/Persistence/PersistentObjectFactory.php b/src/Persistence/PersistentObjectFactory.php
index aee598a16..c4174199a 100644
--- a/src/Persistence/PersistentObjectFactory.php
+++ b/src/Persistence/PersistentObjectFactory.php
@@ -274,7 +274,10 @@ protected function normalizeParameter(string $field, mixed $value): mixed
             $value->persist = $this->persist; // todo - breaks immutability
         }
 
-        if ($value instanceof self && Configuration::instance()->persistence()->relationshipMetadata(static::class(), $value::class(), $field)?->isCascadePersist) {
+        if ($value instanceof self
+            && !Configuration::instance()->isInMemoryEnabled()
+            && Configuration::instance()->persistence()->relationshipMetadata(static::class(), $value::class(), $field)?->isCascadePersist
+        ) {
             $value->persist = false;
         }
 
diff --git a/src/ZenstruckFoundryBundle.php b/src/ZenstruckFoundryBundle.php
index a0f8921c3..9690ed6c5 100644
--- a/src/ZenstruckFoundryBundle.php
+++ b/src/ZenstruckFoundryBundle.php
@@ -12,11 +12,15 @@
 namespace Zenstruck\Foundry;
 
 use Symfony\Component\Config\Definition\Configurator\DefinitionConfigurator;
+use Symfony\Component\DependencyInjection\ChildDefinition;
 use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
 use Symfony\Component\DependencyInjection\Reference;
 use Symfony\Component\HttpKernel\Bundle\AbstractBundle;
+use Zenstruck\Foundry\InMemory\AsInMemoryRepository;
+use Zenstruck\Foundry\InMemory\DependencyInjection\InMemoryCompilerPass;
+use Zenstruck\Foundry\InMemory\InMemoryRepository;
 use Zenstruck\Foundry\Object\Instantiator;
 use Zenstruck\Foundry\ORM\AbstractORMPersistenceStrategy;
 
@@ -211,6 +215,18 @@ public function loadExtension(array $config, ContainerConfigurator $configurator
                 ->replaceArgument(1, $config['mongo'])
             ;
         }
+
+        // tag with "foundry.in_memory.repository" all classes using attribute "AsInMemoryRepository"
+        $container->registerAttributeForAutoconfiguration(
+            AsInMemoryRepository::class,
+            static function (ChildDefinition $definition, AsInMemoryRepository $attribute, \ReflectionClass $reflector) { // @phpstan-ignore-line
+                if (!is_a($reflector->name, InMemoryRepository::class, true)) {
+                    throw new \LogicException(sprintf("Service \"%s\" with attribute \"AsInMemoryRepository\" must implement \"%s\".", $reflector->name, InMemoryRepository::class));
+                }
+
+                $definition->addTag('foundry.in_memory.repository', ['class' => $attribute->class]);
+            }
+        );
     }
 
     public function build(ContainerBuilder $container): void
@@ -218,6 +234,9 @@ public function build(ContainerBuilder $container): void
         parent::build($container);
 
         $container->addCompilerPass($this);
+
+        // todo: should we find a way to decouple Foundry from its "plugins"?
+        $container->addCompilerPass(new InMemoryCompilerPass());
     }
 
     public function process(ContainerBuilder $container): void
diff --git a/tests/Fixture/InMemory/InMemoryStandardAddressRepository.php b/tests/Fixture/InMemory/InMemoryStandardAddressRepository.php
new file mode 100644
index 000000000..7bceb9dbd
--- /dev/null
+++ b/tests/Fixture/InMemory/InMemoryStandardAddressRepository.php
@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\Tests\Fixture\InMemory;
+
+use Zenstruck\Foundry\InMemory\AsInMemoryRepository;
+use Zenstruck\Foundry\InMemory\InMemoryRepository;
+use Zenstruck\Foundry\Tests\Fixture\Entity\Address\StandardAddress;
+
+/**
+ * @implements InMemoryRepository<StandardAddress>
+ */
+#[AsInMemoryRepository(class: StandardAddress::class)]
+final class InMemoryStandardAddressRepository implements InMemoryRepository
+{
+    /**
+     * @var list<StandardAddress>
+     */
+    private array $elements = [];
+
+    public function _save(object $element): void
+    {
+        if (!in_array($element, $this->elements, true)) {
+            $this->elements[] = $element;
+        }
+    }
+
+    /**
+     * @return list<StandardAddress>
+     */
+    public function all(): array
+    {
+        return $this->elements;
+    }
+}
diff --git a/tests/Fixture/InMemory/InMemoryStandardContactRepository.php b/tests/Fixture/InMemory/InMemoryStandardContactRepository.php
new file mode 100644
index 000000000..95d0e21dd
--- /dev/null
+++ b/tests/Fixture/InMemory/InMemoryStandardContactRepository.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\Tests\Fixture\InMemory;
+
+use Zenstruck\Foundry\InMemory\AsInMemoryRepository;
+use Zenstruck\Foundry\InMemory\InMemoryRepository;
+use Zenstruck\Foundry\Tests\Fixture\Entity\Contact\StandardContact;
+
+/**
+ * @implements InMemoryRepository<StandardContact>
+ */
+#[AsInMemoryRepository(class: StandardContact::class)]
+final class InMemoryStandardContactRepository implements InMemoryRepository
+{
+    /** @var list<StandardContact> */
+    private array $elements = [];
+
+    public function _save(object $element): void
+    {
+        if (!in_array($element, $this->elements, true)) {
+            $this->elements[] = $element;
+        }
+    }
+
+    /**
+     * @return list<StandardContact>
+     */
+    public function all(): array
+    {
+        return $this->elements;
+    }
+}
diff --git a/tests/Fixture/TestKernel.php b/tests/Fixture/TestKernel.php
index afa3ae355..87fba5a8e 100644
--- a/tests/Fixture/TestKernel.php
+++ b/tests/Fixture/TestKernel.php
@@ -26,6 +26,8 @@
 use Zenstruck\Foundry\ORM\AbstractORMPersistenceStrategy;
 use Zenstruck\Foundry\Tests\Fixture\Factories\ArrayFactory;
 use Zenstruck\Foundry\Tests\Fixture\Factories\Object1Factory;
+use Zenstruck\Foundry\Tests\Fixture\InMemory\InMemoryStandardAddressRepository;
+use Zenstruck\Foundry\Tests\Fixture\InMemory\InMemoryStandardContactRepository;
 use Zenstruck\Foundry\Tests\Fixture\Stories\GlobalInvokableService;
 use Zenstruck\Foundry\Tests\Fixture\Stories\GlobalStory;
 use Zenstruck\Foundry\ZenstruckFoundryBundle;
@@ -145,6 +147,8 @@ protected function configureContainer(ContainerBuilder $c, LoaderInterface $load
         $c->register(GlobalInvokableService::class);
         $c->register(ArrayFactory::class)->setAutowired(true)->setAutoconfigured(true);
         $c->register(Object1Factory::class)->setAutowired(true)->setAutoconfigured(true);
+        $c->register(InMemoryStandardAddressRepository::class)->setAutowired(true)->setAutoconfigured(true);
+        $c->register(InMemoryStandardContactRepository::class)->setAutowired(true)->setAutoconfigured(true);
     }
 
     protected function configureRoutes(RoutingConfigurator $routes): void
diff --git a/tests/Integration/InMemory/InMemoryTest.php b/tests/Integration/InMemory/InMemoryTest.php
new file mode 100644
index 000000000..e69055849
--- /dev/null
+++ b/tests/Integration/InMemory/InMemoryTest.php
@@ -0,0 +1,99 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zenstruck\Foundry\Tests\Integration\InMemory;
+
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+use Zenstruck\Foundry\Test\Factories;
+use Zenstruck\Foundry\Test\ResetDatabase;
+use Zenstruck\Foundry\Tests\Fixture\Entity\Address\StandardAddress;
+use Zenstruck\Foundry\Tests\Fixture\Factories\Entity\Address\StandardAddressFactory;
+use Zenstruck\Foundry\Tests\Fixture\Factories\Entity\Category\StandardCategoryFactory;
+use Zenstruck\Foundry\Tests\Fixture\Factories\Entity\Contact\StandardContactFactory;
+use Zenstruck\Foundry\Tests\Fixture\InMemory\InMemoryStandardAddressRepository;
+use Zenstruck\Foundry\Tests\Fixture\InMemory\InMemoryStandardContactRepository;
+use function Zenstruck\Foundry\InMemory\enable_in_memory;
+
+final class InMemoryTest extends KernelTestCase
+{
+    use Factories;
+    use ResetDatabase;
+
+    private InMemoryStandardAddressRepository $addressRepository;
+    private InMemoryStandardContactRepository $contactRepository;
+
+    protected function setUp(): void
+    {
+        enable_in_memory();
+
+        $this->addressRepository = self::getContainer()->get(InMemoryStandardAddressRepository::class); // @phpstan-ignore-line
+        $this->contactRepository = self::getContainer()->get(InMemoryStandardContactRepository::class); // @phpstan-ignore-line
+    }
+
+    /**
+     * @test
+     */
+    public function create_one_does_not_persist_in_database(): void
+    {
+        $address = StandardAddressFactory::createOne();
+        self::assertInstanceOf(StandardAddress::class, $address);
+
+        // todo!
+//        StandardAddressFactory::assert()->count(0);
+
+        // id is autogenerated from the db, then it should be null
+        self::assertNull($address->id);
+    }
+
+    /**
+     * @test
+     */
+    public function create_many_does_not_persist_in_database(): void
+    {
+        $addresses = StandardAddressFactory::createMany(2);
+        self::assertContainsOnlyInstancesOf(StandardAddress::class, $addresses);
+
+        // todo!
+//        StandardAddressFactory::assert()->count(0);
+
+        foreach ($addresses as $address) {
+            // id is autogenerated from the db, then it should be null
+            self::assertNull($address->id);
+        }
+    }
+
+    /**
+     * @test
+     */
+    public function object_should_be_accessible_from_in_memory_repository(): void
+    {
+        $address = StandardAddressFactory::createOne();
+
+        self::assertSame([$address], $this->addressRepository->all());
+    }
+
+    /**
+     * @test
+     */
+    public function nested_objects_should_be_accessible_from_their_respective_repository(): void
+    {
+        $contact = StandardContactFactory::createOne();
+
+        self::assertSame([$contact], $this->contactRepository->all());
+        self::assertSame([$contact->getAddress()], $this->addressRepository->all());
+    }
+
+    /**
+     * @test
+     */
+    public function can_use_generic_repository(): void
+    {
+        $category = StandardCategoryFactory::createOne();
+
+        // todo!
+//        StandardCategoryFactory::assert()->count(0);
+
+        self::assertNull($category->id);
+    }
+}