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Tracing is a feature that helps you trace and monitor the execution of your Web pages.
The following links explain more about this feature:
The Basics of ASP.NET Tracing
This is an old MSDN link but still conveys the gist very well.
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
From MSDN Magazine:
How Do I implement tracing in an ASP.NET Web site?
Debugging and Tracing