diff --git a/README.md b/README.md
index f54f2a2..89cae14 100644
--- a/README.md
+++ b/README.md
@@ -441,6 +441,43 @@ Or traditionally:
 $this->addParameterSet(new PaginationParams());
 ```
 
+## Cross-Field Validation
+
+For validation rules that depend on multiple parameters together, use the `#[Validate]` attribute or override the `validate()` method:
+
+### Method-Specific Validation (Attribute)
+
+```php
+#[PostMapping]
+#[ResponseBody]
+#[Validate('validateRegistration')]
+#[RequestParam('password', ParamType::STRING)]
+#[RequestParam('password_confirm', ParamType::STRING)]
+public function register(string $password, string $passwordConfirm): array { ... }
+
+private function validateRegistration(array $inputs): array {
+    $errors = [];
+    if ($inputs['password'] !== $inputs['password_confirm']) {
+        $errors['password_confirm'] = 'Passwords do not match.';
+    }
+    return $errors; // empty = pass
+}
+```
+
+### Service-Wide Validation (Override)
+
+```php
+public function validate(array $inputs): array {
+    $errors = [];
+    if (isset($inputs['end_date']) && $inputs['end_date'] <= $inputs['start_date']) {
+        $errors['end_date'] = 'End date must be after start date.';
+    }
+    return $errors;
+}
+```
+
+Both run if defined — service-wide first, then method-specific. Errors are merged. If any errors exist, the request returns 422 with the error details.
+
 ## Dynamic Status Codes with ResponseEntity
 
 The `ResponseEntity` class allows `#[ResponseBody]` methods to return different HTTP status codes based on runtime logic:
diff --git a/WebFiori/Http/Annotations/Validate.php b/WebFiori/Http/Annotations/Validate.php
new file mode 100644
index 0000000..8bd20bd
--- /dev/null
+++ b/WebFiori/Http/Annotations/Validate.php
@@ -0,0 +1,41 @@
+<?php
+
+/**
+ * This file is licensed under MIT License.
+ * 
+ * Copyright (c) 2026-present WebFiori Framework
+ * 
+ * For more information on the license, please visit: 
+ * https://github.com/WebFiori/.github/blob/main/LICENSE
+ */
+namespace WebFiori\Http\Annotations;
+
+use Attribute;
+
+/**
+ * Specifies a method-specific cross-field validation function.
+ * 
+ * The referenced method must exist on the service class, accept an array
+ * of filtered inputs, and return an array of errors (empty = valid).
+ * 
+ * Usage:
+ * ```php
+ * #[Validate('validateRegistration')]
+ * public function register(...): array { ... }
+ * 
+ * private function validateRegistration(array $inputs): array {
+ *     $errors = [];
+ *     if ($inputs['password'] !== $inputs['password_confirm']) {
+ *         $errors['password_confirm'] = 'Passwords do not match.';
+ *     }
+ *     return $errors;
+ * }
+ * ```
+ */
+#[Attribute(Attribute::TARGET_METHOD)]
+class Validate {
+    public function __construct(
+        public readonly string $method
+    ) {
+    }
+}
diff --git a/WebFiori/Http/WebService.php b/WebFiori/Http/WebService.php
index 29f5f2a..4c028e1 100644
--- a/WebFiori/Http/WebService.php
+++ b/WebFiori/Http/WebService.php
@@ -735,6 +735,21 @@ public static function isValidName(string $name): bool {
      */
     public function processRequest() {
     }
+    /**
+     * Service-wide cross-field validation hook.
+     * 
+     * Override this method to add validation rules that depend on multiple
+     * parameters together. Called after individual parameter validation passes
+     * but before the request method is invoked.
+     * 
+     * @param array $inputs The filtered input values.
+     * 
+     * @return array An associative array of errors keyed by field name.
+     *               Return empty array if validation passes.
+     */
+    public function validate(array $inputs): array {
+        return [];
+    }
 
     /**
      * Process the web service request with auto-processing support.
@@ -752,6 +767,17 @@ public function processWithAutoHandling(): void {
             }
 
             try {
+                // Run cross-field validation
+                $validationErrors = $this->runValidation($targetMethod);
+
+                if (!empty($validationErrors)) {
+                    $this->sendResponse('Validation failed', 422, 'error', new \WebFiori\Json\Json([
+                        'errors' => $validationErrors
+                    ]));
+
+                    return;
+                }
+
                 // Inject parameters into method call
                 $params = $this->getMethodParameters($targetMethod);
                 $result = $this->$targetMethod(...$params);
@@ -1326,6 +1352,54 @@ private function configureParametersForHttpMethod(string $httpMethod): void {
         }
     }
 
+    /**
+     * Runs cross-field validation: service-wide validate() + method-specific #[Validate].
+     * 
+     * @param string $targetMethod The method being invoked.
+     * 
+     * @return array Merged errors from both validators. Empty if all pass.
+     */
+    private function runValidation(string $targetMethod): array {
+        $inputs = $this->getInputs();
+
+        if ($inputs instanceof \WebFiori\Json\Json) {
+            $inputsArray = [];
+
+            foreach ($inputs->getPropsNames() as $name) {
+                $inputsArray[$name] = $inputs->get($name);
+            }
+        } else {
+            $inputsArray = is_array($inputs) ? $inputs : [];
+        }
+
+        // 1. Service-wide validation
+        $errors = $this->validate($inputsArray);
+
+        // 2. Method-specific #[Validate] attribute
+        $reflection = new \ReflectionMethod($this, $targetMethod);
+        $validateAttrs = $reflection->getAttributes(Annotations\Validate::class);
+
+        if (!empty($validateAttrs)) {
+            $validateAnnotation = $validateAttrs[0]->newInstance();
+            $validatorMethod = $validateAnnotation->method;
+
+            if (!method_exists($this, $validatorMethod)) {
+                throw new \InvalidArgumentException(
+                    "Validation method '$validatorMethod' referenced by #[Validate] does not exist on " . get_class($this)
+                );
+            }
+
+            $validatorReflection = new \ReflectionMethod($this, $validatorMethod);
+            $validatorReflection->setAccessible(true);
+            $methodErrors = $validatorReflection->invoke($this, $inputsArray);
+
+            if (is_array($methodErrors)) {
+                $errors = array_merge($errors, $methodErrors);
+            }
+        }
+
+        return $errors;
+    }
     /**
      * Configure parameters from method RequestParam annotations.
      */
diff --git a/tests/WebFiori/Tests/Http/CrossFieldValidationTest.php b/tests/WebFiori/Tests/Http/CrossFieldValidationTest.php
new file mode 100644
index 0000000..b03ed7c
--- /dev/null
+++ b/tests/WebFiori/Tests/Http/CrossFieldValidationTest.php
@@ -0,0 +1,186 @@
+<?php
+namespace WebFiori\Tests\Http;
+
+use WebFiori\Http\Test\ServiceTestCase;
+use WebFiori\Tests\Http\TestServices\ValidationHookService;
+use WebFiori\Tests\Http\TestServices\ServiceWideValidationService;
+
+/**
+ * Tests for cross-field validation hook (#115).
+ * 
+ * Covers:
+ * - Service-wide validate() method
+ * - Method-specific #[Validate] attribute
+ * - Both running together with merged errors
+ * - Missing validator method throws exception
+ * - Validation passes → method invoked
+ */
+class CrossFieldValidationTest extends ServiceTestCase {
+
+    // =========================================================================
+    // Happy path — validation passes
+    // =========================================================================
+
+    public function testValidationPassesAllowsExecution() {
+        $this->post(new ValidationHookService(), [
+            'email' => 'user@example.com',
+            'password' => 'securepass123',
+            'password_confirm' => 'securepass123',
+        ])
+            ->assertOk()
+            ->assertJsonHas('data');
+    }
+
+    public function testServiceWideValidationPasses() {
+        $this->post(new ServiceWideValidationService(), [
+            'start' => '1',
+            'end' => '10',
+        ])
+            ->assertOk()
+            ->assertJsonHas('data');
+    }
+
+    // =========================================================================
+    // Method-specific #[Validate] failures
+    // =========================================================================
+
+    public function testPasswordMismatchFails() {
+        $this->post(new ValidationHookService(), [
+            'email' => 'user@example.com',
+            'password' => 'securepass123',
+            'password_confirm' => 'different',
+        ])
+            ->assertStatus(422)
+            ->assertError()
+            ->assertJsonHas('more-info');
+    }
+
+    public function testPasswordTooShortFails() {
+        $this->post(new ValidationHookService(), [
+            'email' => 'user@example.com',
+            'password' => 'short',
+            'password_confirm' => 'short',
+        ])
+            ->assertStatus(422)
+            ->assertError();
+    }
+
+    // =========================================================================
+    // Service-wide validate() failures
+    // =========================================================================
+
+    public function testServiceWideValidationRejectsInvalidInput() {
+        $this->post(new ServiceWideValidationService(), [
+            'start' => '10',
+            'end' => '5',
+        ])
+            ->assertStatus(422)
+            ->assertError();
+    }
+
+    public function testServiceWidePlusAddressingRejected() {
+        $this->post(new ValidationHookService(), [
+            'email' => 'user%2Btag@example.com',
+            'password' => 'securepass123',
+            'password_confirm' => 'securepass123',
+        ])
+            ->assertStatus(422)
+            ->assertError();
+    }
+
+    // =========================================================================
+    // Both validators run — errors merged
+    // =========================================================================
+
+    public function testBothValidatorsRunErrorsMerged() {
+        $this->post(new ValidationHookService(), [
+            'email' => 'user%2Btag@example.com',
+            'password' => 'short',
+            'password_confirm' => 'different',
+        ])
+            ->assertStatus(422)
+            ->assertError();
+    }
+
+    // =========================================================================
+    // Missing validator method
+    // =========================================================================
+
+    public function testMissingValidatorMethodThrowsException() {
+        $service = new class extends \WebFiori\Http\WebService {
+            public function __construct() {
+                parent::__construct('bad-validator');
+                $this->addRequestMethod('POST');
+            }
+            #[\WebFiori\Http\Annotations\PostMapping]
+            #[\WebFiori\Http\Annotations\ResponseBody]
+            #[\WebFiori\Http\Annotations\AllowAnonymous]
+            #[\WebFiori\Http\Annotations\Validate('nonExistentMethod')]
+            #[\WebFiori\Http\Annotations\RequestParam('name', 'string')]
+            public function doSomething(string $name): array {
+                return ['name' => $name];
+            }
+            public function isAuthorized(): bool { return true; }
+            public function processRequest() {}
+        };
+
+        $response = $this->post($service, ['name' => 'test']);
+        $response->assertBodyContains('nonExistentMethod')
+            ->assertBodyContains('error');
+    }
+
+    // =========================================================================
+    // No validation defined — works normally
+    // =========================================================================
+
+    public function testNoValidationDefinedWorksNormally() {
+        $service = new class extends \WebFiori\Http\WebService {
+            public function __construct() {
+                parent::__construct('no-validation');
+                $this->addRequestMethod('POST');
+            }
+            #[\WebFiori\Http\Annotations\PostMapping]
+            #[\WebFiori\Http\Annotations\ResponseBody]
+            #[\WebFiori\Http\Annotations\AllowAnonymous]
+            #[\WebFiori\Http\Annotations\RequestParam('value', 'string')]
+            public function store(string $value): array {
+                return ['stored' => $value];
+            }
+            public function isAuthorized(): bool { return true; }
+            public function processRequest() {}
+        };
+
+        $this->post($service, ['value' => 'hello'])
+            ->assertOk()
+            ->assertJsonHas('data');
+    }
+
+    // =========================================================================
+    // Validate returns empty array — passes
+    // =========================================================================
+
+    public function testValidateReturnsEmptyArrayPasses() {
+        $service = new class extends \WebFiori\Http\WebService {
+            public function __construct() {
+                parent::__construct('empty-validate');
+                $this->addRequestMethod('POST');
+            }
+            public function validate(array $inputs): array {
+                return []; // Always passes
+            }
+            #[\WebFiori\Http\Annotations\PostMapping]
+            #[\WebFiori\Http\Annotations\ResponseBody]
+            #[\WebFiori\Http\Annotations\AllowAnonymous]
+            #[\WebFiori\Http\Annotations\RequestParam('x', 'string')]
+            public function process(string $x): array {
+                return ['x' => $x];
+            }
+            public function isAuthorized(): bool { return true; }
+            public function processRequest() {}
+        };
+
+        $this->post($service, ['x' => 'test'])
+            ->assertOk()
+            ->assertJsonHas('data');
+    }
+}
diff --git a/tests/WebFiori/Tests/Http/TestServices/ServiceWideValidationService.php b/tests/WebFiori/Tests/Http/TestServices/ServiceWideValidationService.php
new file mode 100644
index 0000000..f95592e
--- /dev/null
+++ b/tests/WebFiori/Tests/Http/TestServices/ServiceWideValidationService.php
@@ -0,0 +1,44 @@
+<?php
+namespace WebFiori\Tests\Http\TestServices;
+
+use WebFiori\Http\Annotations\AllowAnonymous;
+use WebFiori\Http\Annotations\PostMapping;
+use WebFiori\Http\Annotations\RequestParam;
+use WebFiori\Http\Annotations\ResponseBody;
+use WebFiori\Http\Annotations\RestController;
+use WebFiori\Http\Annotations\Validate;
+use WebFiori\Http\ParamType;
+use WebFiori\Http\WebService;
+
+/**
+ * Service with only service-wide validate() — no #[Validate] attribute.
+ */
+#[RestController('service-wide-validation')]
+class ServiceWideValidationService extends WebService {
+
+    public function validate(array $inputs): array {
+        $errors = [];
+
+        if (isset($inputs['start']) && isset($inputs['end']) && $inputs['end'] <= $inputs['start']) {
+            $errors['end'] = 'End must be after start.';
+        }
+
+        return $errors;
+    }
+
+    #[PostMapping]
+    #[ResponseBody]
+    #[AllowAnonymous]
+    #[RequestParam('start', ParamType::INT)]
+    #[RequestParam('end', ParamType::INT)]
+    public function createRange(int $start, int $end): array {
+        return ['start' => $start, 'end' => $end];
+    }
+
+    public function isAuthorized(): bool {
+        return true;
+    }
+
+    public function processRequest() {
+    }
+}
diff --git a/tests/WebFiori/Tests/Http/TestServices/ValidationHookService.php b/tests/WebFiori/Tests/Http/TestServices/ValidationHookService.php
new file mode 100644
index 0000000..a3a9c0a
--- /dev/null
+++ b/tests/WebFiori/Tests/Http/TestServices/ValidationHookService.php
@@ -0,0 +1,63 @@
+<?php
+namespace WebFiori\Tests\Http\TestServices;
+
+use WebFiori\Http\Annotations\AllowAnonymous;
+use WebFiori\Http\Annotations\PostMapping;
+use WebFiori\Http\Annotations\RequestParam;
+use WebFiori\Http\Annotations\ResponseBody;
+use WebFiori\Http\Annotations\RestController;
+use WebFiori\Http\Annotations\Validate;
+use WebFiori\Http\ParamType;
+use WebFiori\Http\WebService;
+
+#[RestController('validation-hook-service')]
+class ValidationHookService extends WebService {
+
+    /**
+     * Service-wide validation: applies to all methods.
+     */
+    public function validate(array $inputs): array {
+        $errors = [];
+
+        if (isset($inputs['email']) && str_contains($inputs['email'], '+')) {
+            $errors['email'] = 'Plus-addressing not allowed.';
+        }
+
+        return $errors;
+    }
+
+    #[PostMapping]
+    #[ResponseBody]
+    #[AllowAnonymous]
+    #[Validate('validateRegistration')]
+    #[RequestParam('email', ParamType::EMAIL)]
+    #[RequestParam('password', ParamType::STRING)]
+    #[RequestParam('password_confirm', ParamType::STRING)]
+    public function register(string $email, string $password, string $passwordConfirm): array {
+        return ['registered' => true, 'email' => $email];
+    }
+
+    /**
+     * Method-specific validator for register().
+     */
+    private function validateRegistration(array $inputs): array {
+        $errors = [];
+
+        if ($inputs['password'] !== $inputs['password_confirm']) {
+            $errors['password_confirm'] = 'Passwords do not match.';
+        }
+
+        if (strlen($inputs['password']) < 8) {
+            $errors['password'] = 'Password must be at least 8 characters.';
+        }
+
+        return $errors;
+    }
+
+    public function isAuthorized(): bool {
+        return true;
+    }
+
+    public function processRequest() {
+    }
+}