WCF service using authentication - membership need certificate?

Hi. I am able to use WCF services but i have one question. I am looking at securing the WCF using authentication and membership database when data is passed to an asp.net application. My question is, do i need to add a certificate to the server in order for the membership to work? Right now i can pass the username-password combination but no matter what credentials i may give the service will always go through unblocked. So do i have to use a certificate or is there another way?
Thanks.