You can add references to assembly namespaces in your web.config to avoid having to register controls on every .aspx in your web site:

If you have a custom control located in your site's App_Code folder however, the assembly name is generated at compile-time. If you want to reference controls in the App_Code folder, just use "App_Code" as the assembly name:

You can also choose to leave out the assembly attribute entirely - the compiler will try find the namespace in the App_Code assembly by default. I prefer to specify "App_Code" to eliminate potential confusion as to where  the control is located. 

Jimmy