diff --git a/packages/core/composer.json b/packages/core/composer.json
index 61182e788..cf906e8d3 100644
--- a/packages/core/composer.json
+++ b/packages/core/composer.json
@@ -13,6 +13,10 @@
         "symfony/cache": "^7.3",
         "filp/whoops": "^2.15"
     },
+    "require-dev": {
+        "tempest/validation": "dev-main",
+        "tempest/intl": "dev-main"
+    },
     "autoload": {
         "psr-4": {
             "Tempest\\Core\\": "src"
diff --git a/packages/core/src/EnvironmentVariableValidationFailed.php b/packages/core/src/EnvironmentVariableValidationFailed.php
new file mode 100644
index 000000000..1bb4f4d22
--- /dev/null
+++ b/packages/core/src/EnvironmentVariableValidationFailed.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace Tempest\Core;
+
+use Exception;
+use Tempest\Validation\FailingRule;
+use Tempest\Validation\Validator;
+
+use function Tempest\Support\arr;
+
+final class EnvironmentVariableValidationFailed extends Exception
+{
+    /**
+     * @param FailingRule[] $failingRules
+     */
+    public function __construct(
+        private(set) string $name,
+        private(set) mixed $value,
+        private(set) array $failingRules,
+        private(set) Validator $validator,
+    ) {
+        return parent::__construct(vsprintf("Environment variable [%s] is not valid:\n- %s", [
+            $name,
+            arr($failingRules)
+                ->map(fn (FailingRule $failingRule) => $validator->getErrorMessage($failingRule, $name))
+                ->implode("\n- ")
+                ->toString(),
+        ]));
+    }
+}
diff --git a/packages/core/src/functions.php b/packages/core/src/functions.php
index 4c258d29f..ac94a15d2 100644
--- a/packages/core/src/functions.php
+++ b/packages/core/src/functions.php
@@ -7,9 +7,13 @@
     use Stringable;
     use Tempest\Core\Composer;
     use Tempest\Core\DeferredTasks;
+    use Tempest\Core\EnvironmentVariableValidationFailed;
     use Tempest\Core\ExceptionReporter;
     use Tempest\Core\Kernel;
+    use Tempest\Intl\Translator;
     use Tempest\Support\Namespace\PathCouldNotBeMappedToNamespace;
+    use Tempest\Validation\Rule;
+    use Tempest\Validation\Validator;
     use Throwable;
 
     use function Tempest\Support\Namespace\to_psr4_namespace;
@@ -61,21 +65,36 @@ function src_namespace(Stringable|string ...$parts): string
 
     /**
      * Retrieves the given `$key` from the environment variables. If `$key` is not defined, `$default` is returned instead.
+     *
+     * @param Rule[] $rules Optional validation rules for the value of this environment variable. If one of the rules don't pass, an exception is thrown, preventing the application from booting.
      */
-    function env(string $key, mixed $default = null): mixed
+    function env(string $key, mixed $default = null, array $rules = []): mixed
     {
         $value = getenv($key);
-
-        if ($value === false) {
-            return $default;
-        }
-
-        return match (strtolower($value)) {
+        $value = match (is_string($value) ? mb_strtolower($value) : $value) {
             'true' => true,
             'false' => false,
-            'null', '' => null,
+            false, 'null', '' => $default,
             default => $value,
         };
+
+        if ($rules === [] || ! class_exists(Validator::class) || ! class_exists(Translator::class)) {
+            return $value;
+        }
+
+        $validator = get(Validator::class);
+        $failures = $validator->validateValue($value, $rules);
+
+        if ($failures === []) {
+            return $value;
+        }
+
+        throw new EnvironmentVariableValidationFailed(
+            name: $key,
+            value: $value,
+            failingRules: $failures,
+            validator: $validator,
+        );
     }
 
     /**
diff --git a/packages/core/tests/EnvTest.php b/packages/core/tests/EnvTest.php
new file mode 100644
index 000000000..b11765fff
--- /dev/null
+++ b/packages/core/tests/EnvTest.php
@@ -0,0 +1,120 @@
+<?php
+
+namespace Tempest\Core\Tests;
+
+use PHPUnit\Framework\Attributes\PreCondition;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\TestWith;
+use PHPUnit\Framework\TestCase;
+use Tempest\Container\GenericContainer;
+use Tempest\Core\EnvironmentVariableValidationFailed;
+use Tempest\Intl\Catalog\GenericCatalog;
+use Tempest\Intl\GenericTranslator;
+use Tempest\Intl\IntlConfig;
+use Tempest\Intl\Locale;
+use Tempest\Intl\MessageFormat\Formatter\MessageFormatter;
+use Tempest\Intl\Translator;
+use Tempest\Validation\Rules\IsBoolean;
+use Tempest\Validation\Rules\IsNotNull;
+use Tempest\Validation\Rules\IsNumeric;
+use Tempest\Validation\Validator;
+
+use function Tempest\env;
+
+final class EnvTest extends TestCase
+{
+    #[PreCondition]
+    protected function configure(): void
+    {
+        if (! class_exists(Translator::class)) {
+            $this->markTestSkipped('`tempest/intl` is required for this test.');
+        }
+
+        if (! class_exists(Validator::class)) {
+            $this->markTestSkipped('`tempest/validation` is required for this test.');
+        }
+
+        $container = new GenericContainer();
+        $container->singleton(Translator::class, new GenericTranslator(
+            config: new IntlConfig(currentLocale: Locale::ENGLISH, fallbackLocale: Locale::ENGLISH),
+            catalog: new GenericCatalog([
+                'en' => [
+                    'validation_error' => [
+                        'is_numeric' => '{{{$field} must be a numeric value}}',
+                    ],
+                ],
+            ]),
+            formatter: new MessageFormatter(),
+        ));
+
+        GenericContainer::setInstance($container);
+    }
+
+    #[Test]
+    #[TestWith([null, null])]
+    #[TestWith(['', null])]
+    #[TestWith(['null', null])]
+    #[TestWith([false, null])]
+    #[TestWith(['FALSE', false])]
+    #[TestWith(['false', false])]
+    #[TestWith(['TRUE', true])]
+    #[TestWith(['true', true])]
+    #[TestWith(['foo', 'foo'])]
+    #[TestWith(['FOO', 'FOO'])]
+    #[TestWith([1, '1'])]
+    public function basic(mixed $value, mixed $expected): void
+    {
+        putenv("_ENV_TESTING_KEY={$value}");
+
+        $this->assertSame($expected, env('_ENV_TESTING_KEY'));
+    }
+
+    #[Test]
+    #[TestWith([null, 'fallback', 'fallback'])]
+    #[TestWith([false, 'fallback', 'fallback'])]
+    #[TestWith(['', 'fallback', 'fallback'])]
+    #[TestWith(['false', 'fallback', false])]
+    #[TestWith(['true', 'fallback', true])]
+    #[TestWith([false, '', ''])]
+    #[TestWith([null, '', ''])]
+    #[TestWith(['', '', ''])]
+    #[TestWith([false, false, false])]
+    #[TestWith([null, false, false])]
+    #[TestWith(['', false, false])]
+    public function default(mixed $value, mixed $default, mixed $expected): void
+    {
+        putenv("_ENV_TESTING_KEY={$value}");
+
+        $this->assertSame($expected, env('_ENV_TESTING_KEY', default: $default));
+    }
+
+    #[Test]
+    public function fails_with_failing_rules(): void
+    {
+        $this->expectException(EnvironmentVariableValidationFailed::class);
+        $this->expectExceptionMessageMatches('*_ENV_TESTING_KEY must be a numeric value*');
+
+        putenv('_ENV_TESTING_KEY=foo');
+        env('_ENV_TESTING_KEY', rules: [new IsNumeric()]);
+    }
+
+    #[Test]
+    #[TestWith([null, null])]
+    #[TestWith(['', null])]
+    #[TestWith([false, null])]
+    public function default_taken_into_account(mixed $value, mixed $default): void
+    {
+        $this->expectException(EnvironmentVariableValidationFailed::class);
+
+        putenv("_ENV_TESTING_KEY={$value}");
+        env('_ENV_TESTING_KEY', default: $default, rules: [new IsNotNull()]);
+    }
+
+    #[Test]
+    public function can_pass(): void
+    {
+        putenv('_ENV_TESTING_KEY=true');
+
+        $this->assertSame(true, env('_ENV_TESTING_KEY', rules: [new IsBoolean()]));
+    }
+}
diff --git a/packages/validation/src/Exceptions/TranslatorWasRequired.php b/packages/validation/src/Exceptions/TranslatorWasRequired.php
new file mode 100644
index 000000000..0a5acc634
--- /dev/null
+++ b/packages/validation/src/Exceptions/TranslatorWasRequired.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tempest\Validation\Exceptions;
+
+use Exception;
+
+final class TranslatorWasRequired extends Exception
+{
+    public function __construct()
+    {
+        parent::__construct('A translator instance is required to generate validation error messages, but none was provided.');
+    }
+}
diff --git a/packages/validation/src/Validator.php b/packages/validation/src/Validator.php
index 9adf124f3..f753b73f4 100644
--- a/packages/validation/src/Validator.php
+++ b/packages/validation/src/Validator.php
@@ -10,6 +10,7 @@
 use Tempest\Reflection\ClassReflector;
 use Tempest\Reflection\PropertyReflector;
 use Tempest\Support\Arr;
+use Tempest\Validation\Exceptions\TranslatorWasRequired;
 use Tempest\Validation\Exceptions\ValidationFailed;
 use Tempest\Validation\Rules\IsBoolean;
 use Tempest\Validation\Rules\IsEnum;
@@ -25,7 +26,7 @@
 final readonly class Validator
 {
     public function __construct(
-        private Translator $translator,
+        private ?Translator $translator = null,
     ) {}
 
     /**
@@ -213,6 +214,10 @@ public function validateValues(iterable $values, array $rules): array
      */
     public function getErrorMessage(Rule|FailingRule $rule, ?string $field = null): string
     {
+        if (is_null($this->translator)) {
+            throw new TranslatorWasRequired();
+        }
+
         if ($rule instanceof HasErrorMessage) {
             return $rule->getErrorMessage();
         }
@@ -261,7 +266,7 @@ private function getTranslationKey(Rule|FailingRule $rule): string
 
     private function getFieldName(string $key, ?string $field = null): string
     {
-        $translatedField = $this->translator->translate("validation_field.{$key}");
+        $translatedField = $this->translator?->translate("validation_field.{$key}");
 
         if ($translatedField === "validation_field.{$key}") {
             return $field ?? 'Value';