An ASP.NET developer is responsible for designing, implementing, and maintaining web applications using the .NET programming framework. They work closely with business analysts and designers to develop functional and effective software. Their tasks include writing clean, scalable code using .NET programming languages, testing and deploying applications and systems, revising, updating, and debugging code to improve application performance, and developing technical interfaces, specifications, and architecture. They also use ASP.NET MVC to create server-side web applications, making them responsible for application responsiveness and speed. Furthermore, they utilize Azure Web Services and keep up-to-date with the latest technology trends.