How can I make a procedure (or macro) run every time a workbook is opened?

I've tried naming it
Private Sub Autoexec()
and
Private Sub Startup()

But neither work. I'm sure I've done this before but I just can't remember
how.
I looked for startup options (like in Access) but I can't find where to name
a startup macro.

I don't want to use a switch on the startup line.

If anyone can help I'd really appreciate it.

Thanks,
Rick