How to determine the currently logged on username

Hi all;

I will be writing a stored proceedure that will allow a currently logged 
in user to change his/her password.  The function needs to be only able 
to change the password of the currently logged in user, so it will only 
take a varchar() argument and needs to look up the username of the 
currently logged in user.  How do I do this?  Any ideas?

Best Wishes,
Chris Travers