An ASP.NET developer builds, tests, and deploys web applications and services using the ASP.NET framework. They use languages such as C# and .NET to develop server-side logic, define and maintain databases, and ensure high performance and responsiveness of applications. They're also responsible for integrating front-end elements into applications, creating reusable code and libraries, optimizing applications for speed and scalability, and ensuring data security. Moreover, they collaborate with other team members to understand user requirements, provide technical solutions, and improve functionality. They also debug applications, update existing software, and stay updated with the latest industry trends.