ASP.NET Page Lifecycle
The page execution cycle can be a difficult thing to master. Most issues with an event not firing or form data being lost during postback can usually be solved by better understanding the lifecycle. I've constructed a few targeted searches to help out.
ASP.Net 1.x Page and Control Execution Lifecycle
ASP.Net 2.x Page and Control Execution Lifecycle [Diagram by Léon Andrianarivony]
A few things to look for when events don't fire.
[Updated: 2006-06-27 Added diagram, thanks Ambrose]