Change page depending on whether user is logged in

About 9 months ago I designed some pages for a WordPress site that had different content if the user wasd logged in. The text of the page surounds the text to be shown if the user is _not_ logged in with [not_logged_in]…{/not_logged_in], and the text to be shown when he _is_ logged in by [logged_in]…[/logged_in]. It all works fine on my existing site, but I can’t get it working on a new site, and I can’t remember how I did it… I can’t see any obvious plugin or anything. Can anyone suggest what is making this work?

Thanks – Rowan