Register User Controls and Custom Controls in Web.config

<system.web>
    <pages>
        <controls>
            <add tagPrefix="scottgu" src="~/Controls/Header.ascx" tagName="header"/>
            <add tagPrefix="scottgu" src="~/Controls/Footer.ascx" tagName="footer"/>
            <add tagPrefix="ControlVendor" assembly="ControlVendorAssembly"/>
        </controls>
    </pages>
</system.web>