ASP.NET Tracing

Tracing is a feature that helps you trace and monitor the execution of your Web pages.

The following links explain more about this feature:

Tracing Tutorial
The Basics of ASP.NET Tracing

This is an old MSDN link but still conveys the gist very well.

Tracing

And one from Scott Mitchell is here:

ASP.NET Tracing article at 4guysfromRolla.com 

Here's one on tracing with IIS7.0, which might be useful for those working with .NET 3.5 and ASP.NET 3.5. 

Troubleshooting failed requests with IIS 7.0

Videos:

From MSDN Magazine:

How Do I implement tracing in an ASP.NET Web site?
Debugging and Tracing