Store/assign roles of authenticated users

I am upgrading a site to use MVC and I am looking for the best way to set up Authentication.

At this point, I have the log-in working off of Active Directory: validating a username and password, and then setting the Auth cookie.

How do I store the user's role information at time of log-in, in order for my controllers to see those roles as the user navigates through the site?

[Authorize(Roles = "admin")]

I have no problem getting a list of roles from Active Directory. I just don't know where to put them so that the controllers will see them.