diff --git a/features/not_crashing_behat.feature b/features/not_crashing_behat.feature
index fbad0c3..69f0135 100644
--- a/features/not_crashing_behat.feature
+++ b/features/not_crashing_behat.feature
@@ -3,7 +3,7 @@ Feature: Not crashing Behat
     As a Behat User
     I want to have Behat up and running after enabling this extension
 
-    Scenario: Not crashing Behat
+    Scenario: Successful boot the Symfony kernel with autoconfiguration
         Given a Behat configuration containing:
         """
         default:
@@ -29,7 +29,7 @@ Feature: Not crashing Behat
         When I run Behat
         Then it should pass
 
-    Scenario: Not crashing Behat with CrossContainerExtension
+    Scenario: Successful boot the Symfony kernel with explicit configuration
         Given a Behat configuration containing:
         """
         default:
@@ -37,17 +37,19 @@ Feature: Not crashing Behat
                 FriendsOfBehat\SymfonyExtension:
                     kernel:
                         bootstrap: ~
-
-                FriendsOfBehat\CrossContainerExtension: ~
+                        path: app/MyKernel.php
+                        class: MyKernel
+                        env: test
+                        debug: true
         """
-        And a file "app/AppKernel.php" containing:
+        And a file "app/MyKernel.php" containing:
         """
         <?php
 
         use Symfony\Component\HttpKernel\Kernel;
         use Symfony\Component\Config\Loader\LoaderInterface;
 
-        class AppKernel extends Kernel
+        class MyKernel extends Kernel
         {
             public function registerBundles() { return []; }
             public function registerContainerConfiguration(LoaderInterface $loader) {}
@@ -57,7 +59,38 @@ Feature: Not crashing Behat
         When I run Behat
         Then it should pass
 
-    Scenario: This extension boot a Symfony4 kernel
+
+    Scenario: Successful boot the Symfony 4 kernel with autoconfiguration
+        Given a Behat configuration containing:
+        """
+        default:
+            extensions:
+                FriendsOfBehat\SymfonyExtension: ~
+        """
+        And a file ".env" containing:
+        """
+        APP_ENV=dev
+        """
+        And a file "src/Kernel.php" containing:
+        """
+        <?php
+
+        namespace App;
+
+        use Symfony\Component\HttpKernel\Kernel as BaseKernel;
+        use Symfony\Component\Config\Loader\LoaderInterface;
+
+        class Kernel extends BaseKernel
+        {
+            public function registerBundles() { return []; }
+            public function registerContainerConfiguration(LoaderInterface $loader) {}
+        }
+        """
+        And a feature file with passing scenario
+        When I run Behat
+        Then it should pass
+
+    Scenario: Successful boot the Symfony 4 kernel with explicit configuration
         Given a Behat configuration containing:
         """
         default:
@@ -98,9 +131,11 @@ Feature: Not crashing Behat
                 FriendsOfBehat\SymfonyExtension:
                     env_file: .env_in_memory
                     kernel:
+                        bootstrap: ~
                         path: src/MyKernel.php
                         class: MyKernel
-                        bootstrap: ~
+                        env: dev
+                        debug: true
         """
         And a file ".env_in_memory.dist" containing:
         """
@@ -122,3 +157,31 @@ Feature: Not crashing Behat
         And a feature file with passing scenario
         When I run Behat
         Then it should pass
+
+    Scenario: Not crashing Behat with CrossContainerExtension
+        Given a Behat configuration containing:
+        """
+        default:
+            extensions:
+                FriendsOfBehat\SymfonyExtension:
+                    kernel:
+                        bootstrap: ~
+
+                FriendsOfBehat\CrossContainerExtension: ~
+        """
+        And a file "app/AppKernel.php" containing:
+        """
+        <?php
+
+        use Symfony\Component\HttpKernel\Kernel;
+        use Symfony\Component\Config\Loader\LoaderInterface;
+
+        class AppKernel extends Kernel
+        {
+            public function registerBundles() { return []; }
+            public function registerContainerConfiguration(LoaderInterface $loader) {}
+        }
+        """
+        And a feature file with passing scenario
+        When I run Behat
+        Then it should pass
diff --git a/src/ServiceContainer/SymfonyExtension.php b/src/ServiceContainer/SymfonyExtension.php
index ca0dac1..bceb678 100644
--- a/src/ServiceContainer/SymfonyExtension.php
+++ b/src/ServiceContainer/SymfonyExtension.php
@@ -61,7 +61,35 @@ final class SymfonyExtension implements Extension
     /**
      * Enable or disable the debug mode
      */
-    private const DEBUG_MODE = false;
+    private const DEFAULT_DEBUG_MODE = true;
+
+    /**
+     * Default Symfony configuration
+     */
+    private const SYMFONY_DEFAULTS = [
+        'env_file' => null,
+        'kernel' => [
+            'bootstrap' => 'app/autoload.php',
+            'path' => 'app/AppKernel.php',
+            'class' => 'AppKernel',
+            'env' => self::DEFAULT_ENV,
+            'debug' => self::DEFAULT_DEBUG_MODE
+        ]
+    ];
+
+    /**
+     * Default Symfony 4 configuration
+     */
+    private const SYMFONY_4_DEFAULTS = [
+        'env_file' => '.env',
+        'kernel' => [
+            'bootstrap' => null,
+            'path' => 'src/Kernel.php',
+            'class' => 'App\Kernel',
+            'env' => self::DEFAULT_ENV,
+            'debug' => self::DEFAULT_DEBUG_MODE
+        ]
+    ];
 
     /**
      * @var CrossContainerProcessor|null
@@ -91,18 +119,15 @@ final class SymfonyExtension implements Extension
     public function configure(ArrayNodeDefinition $builder): void
     {
         $builder
-            ->addDefaultsIfNotSet()
-                ->children()
-                    ->scalarNode('env_file')->defaultNull()->end()
-                    ->arrayNode('kernel')
-                        ->addDefaultsIfNotSet()
-                        ->children()
-                            ->scalarNode('bootstrap')->defaultValue('app/autoload.php')->end()
-                            ->scalarNode('path')->defaultValue('app/AppKernel.php')->end()
-                            ->scalarNode('class')->defaultValue('AppKernel')->end()
-                            ->scalarNode('env')->defaultValue('test')->end()
-                            ->booleanNode('debug')->defaultTrue()->end()
-                        ->end()
+            ->children()
+                ->scalarNode('env_file')->end()
+                ->arrayNode('kernel')
+                    ->children()
+                        ->scalarNode('bootstrap')->end()
+                        ->scalarNode('path')->end()
+                        ->scalarNode('class')->end()
+                        ->scalarNode('env')->end()
+                        ->booleanNode('debug')->end()
                     ->end()
                 ->end()
             ->end()
@@ -114,17 +139,7 @@ final class SymfonyExtension implements Extension
      */
     public function load(ContainerBuilder $container, array $config): void
     {
-        if (null !== $config['env_file']) {
-            $envFilePath = sprintf('%s/%s', $container->getParameter('paths.base'), $config['env_file']);
-            $envFilePath = file_exists($envFilePath) ? $envFilePath : $envFilePath.'.dist';
-            (new Dotenv())->load($envFilePath);
-
-            $environment = false !== getenv('APP_ENV') ? getenv('APP_ENV') : self::DEFAULT_ENV;
-            $debugMode = false !== getenv('APP_DEBUG') ? getenv('APP_DEBUG') : self::DEBUG_MODE;
-
-            $config['kernel']['env'] = $environment;
-            $config['kernel']['kernel'] = $debugMode;
-        }
+        $config = $this->autoconfigure($container, $config);
 
         $this->loadKernel($container, $config['kernel']);
         $this->loadKernelContainer($container);
@@ -145,6 +160,47 @@ final class SymfonyExtension implements Extension
     {
     }
 
+    /**
+     * @param ContainerBuilder $container
+     * @param array $userConfig
+     * @return array
+     */
+    private function autoconfigure(ContainerBuilder $container, array $userConfig) {
+
+        $defaults = self::SYMFONY_DEFAULTS;
+
+        $symfonyFlexKernelPath = sprintf('%s/%s', $container->getParameter('paths.base'), self::SYMFONY_4_DEFAULTS['kernel']['path']);
+        if (file_exists($symfonyFlexKernelPath)) {
+            $defaults = self::SYMFONY_4_DEFAULTS;
+        }
+
+        $config = array_replace_recursive($defaults, $userConfig);
+
+        if (null !== $config['env_file']) {
+            $this->loadEnvVars($container, $config['env_file']);
+
+            if (!isset($userConfig['kernel']['env']) && false !== getenv('APP_ENV')) {
+                $config['kernel']['env'] = getenv('APP_ENV');
+            }
+
+            if (!isset($userConfig['kernel']['debug']) && false !== getenv('APP_DEBUG')) {
+                $config['kernel']['debug'] = getenv('APP_DEBUG');
+            }
+        }
+
+        return $config;
+    }
+
+    /**
+     * @param ContainerBuilder $container
+     * @param string $fileName
+     */
+    private function loadEnvVars(ContainerBuilder $container, string $fileName): void
+    {
+        $envFilePath = sprintf('%s/%s', $container->getParameter('paths.base'), $fileName);
+        (new Dotenv())->load($envFilePath);
+    }
+
     /**
      * @param ContainerBuilder $container
      */