It depends on the particular situation. If you're getting an error message, that can help to diagnose the problem. One usual mistake is to place server controls outside a form with runat="server" attribute. This doesn't work. Beware that HTML controls can be regular (not processed on the server) or server controls (right-click it and select Run as Server Control). If you're getting the former but not the latter, then you probably have an ASP.NET registration problem.
If this is the case, you can open the Visual Studio .NET command prompt and run: aspnet_regiis /i. This will register ASP.NET with IIS in all the Web sites you have configured.
This was first published in April 2003