While some participants are dropping out of the Medicare shared savings program of accountable care organizations (ACOs), others continue to step up and commercial payors continue to announce their burgeoning networks of ACOs.  Physicians end up being bombarded with requests to participate with these organizations. There are risks and potential rewards in joining these untested vehicles.  In "What to ask if an ACO comes callingAlice and Dan provide some ammunition to physicians seeking to understand with whom and around what they are dealing.