A GraphQL developer designs and implements APIs using GraphQL, a query language for APIs and runtime for executing those queries. They create robust, secure, and efficient data-driven applications, ensuring optimal performance and scalability. They define and document the data types, queries, and mutations in the GraphQL schema. They work closely with front-end developers to connect UI components with server-side logic, and with back-end developers to design and manage the database. They also write tests, debug applications, and maintain code quality and organization. They may also be involved in data migrations, transformations, and solving complex performance problems.