ASPX is a text file format used to create Webform pages; in programming jargon, the ASPX file usually contains static HTML or XHTML markup, as well as markup defining Web Controls and Web 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 which is like other web development technologies such as PHP, JSP, and ASP, but this practice is generally frowned upon by Microsoft except for the purposes of data binding since it requires more calls when rendering the page.

The method recommended by Microsoft for dealing with dynamic program code is to use the code-behind model, which places this code in a separate file or in a especially designated script tag. Code-behind files are normally named something to the effect of MyPage.aspx.cs or MyPage.aspx.vb based on the ASPX file name (this practice is usual in Microsoft Visual Studio and other IDEs). When using this style of programming, the developer writes code to react to different events, like the page being loaded, or a control being clicked, rather than a procedural walk through the document.

ASP.NET uses a visited composites represented technique. During compilation the template (.aspx) file is compiled into initialization code which will build a control tree (the composite) instead of the original (static) template. Literal text goes into instances of the Literal control class, server controls signifies the instances of a specific control class. The initialization code is combined with user-written and results in a class specific for the page. Actual requests for the page are processed during number of steps. First, during the initialization steps, an instance of the page class is formed and the initialization code is executed. This produces the initial control tree which is now usually manipulated by the methods of the page in the following steps. As each node in the tree is a control represented as an instance of a class, the code may alter the tree structure as well as manipulate the properties/methods of the individual nodes. Finally, during the rendering step a visitor is used to visit each node in the tree, asking each node to render itself using the methods of the visitor. The resultant HTML code is sent to the client.

The ASPX and other resource files are positioned in a virtual host on an Internet Information Services. The initial time a client requests a page, the .NET framework compiles and parses the file(s) into a .NET assembly and sends the response; succeeding requests are served from the dll files.