ASP.NET is a web application framework developed and marketed by Microsoft to allow programmers to build dynamic web sites, web applications and web services. It was first released in January 2002 with version 1.0 of the .NET Framework, and is the successor to Microsoft's Active Server Pages (ASP) technology. ASP.NET is built on the Common Language Runtime (CLR), allowing programmers to write ASP.NET code using any supported .NET language.

.NET pages, known officially as "web forms", are the main building block for application development. Web forms are contained in files with an ".aspx" extension; in programming jargon, these files typically contain static (X)HTML markup, as well as markup defining server-side Web Controls and User Controls where the developers place all the required static and dynamic content for the web page.

Additionally, dynamic code which runs on the server can be placed in a page within a block <% -- dynamic code -- %> which is similar to other web development technologies such as PHP, JSP, and ASP, but this practice is generally discouraged except for the purposes of data binding since it requires more calls when rendering the page