How to register custom server control on ASP.NET page

I have a project and I am trying to register a custom server control (there is no .ascx file) on the page. I am currently using

Class Declaration

namespace MyApp.Controls{
    public class CustomControl: WebControl{
        public string Text
        {
            get
            {
                String s = (String)ViewState["Text"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["Text"] = value;
            }
        }        
        protected override void RenderContents(HtmlTextWriter output)
        {
            output.Write(Text);
        }
    }
}

On my page,

<%@ Register TagPrefix="myControls" Namespace="MyApp.Controls" %>
<myControls:CustomControl runat="server" Text="What up!" />

I receive a Parser Error, with the message "Unknown server tag 'myControls:CustomControl'."

What am I doing wrong?

Well, if this control is in another class library, or even if it's in the same one, it wouldn't be a bad idea to specify control's assembly in @Register:

<%@ Register TagPrefix="myControls" Namespace="MyApp.Controls" Assembly="MyApp" %>
<myControls:CustomControl runat="server" Text="What's up!" />

Clean and rebuild your solution too in order to verify everything is compiled rightly!