What are query parameters in Express.js?

Query parameters are key-value pairs appended to the URL after a ?: /products?category=electronics&sort=price&page=2. Access them in Express via req.query: req.query.category returns "electronics", req.query.page returns "2" (always a string). Multiple values for the same key come through as an array: /filter?color=red&color=blue → req.query.color = ["red", "blue"]. Always validate and sanitize query parameters before use — never trust user input. Parse numbers with parseInt(), validate enums against allowed values, and use libraries like Joi or Zod for comprehensive validation schemas.