How to interact with background.js from the popup?

Is there any way to pass information between the popup.html (or popup.js) and a background script?

I'd like the popup to display information from a user's account or allow them to log in, but have the background.js handle the authentication and other logic. I have background.js declared in the manifest, but there seems to be no indication that it's being used at all.

Edit: If I'm going about this all wrong and there's a better way to do this, that would be great too.