Managing Subscriptions with CI + Paypal

[eluser]NateL[/eluser]
With the Paypal library available, I want to set up a subscriptions system. A person will pay via Paypal, and their account will be updated to get the content they're paying for.

I know Paypal calls back upon a successful transaction, which runs the code to update a persons profile, and they get their subscription perks.

But, what happens when a person cancels their subscription? Does Paypal have any sort of communication with my system? How's that work?

Thanks,
-Nate