Setting Control Tag Visible Property from Code behind

I am trying to set the visible property of a button by calling a code behind method but can not get it to work. I have done it in the past so I know it is possible but do not remeber how. Here is what I have so far:
 
Code Behind
public bool ControlEnabled()
{
if (xxxx==yyyy) return true else return false;
} 
 
then for the button definition I have
<
 
asp:Button ID="Button1" Go to the complete details ...