The Affordable Care Act encourages providers to collaborate to improve health care. But the antitrust enforcement agencies have warned providers that “too much” collaboration can run afoul of the antitrust laws. How do providers form networks (including but not limited to ACOs) to deliver better care while not triggering an expensive government investigation or, worse, litigation? Other networks are formed by insurers—and, since passage of the ACA, many so-called “narrow networks” are being formed that exclude willing providers. Does the exclusion of providers from these networks raise antitrust issues?