This chapter discusses healthcare provision in the United States and European Union, setting out some of the challenges faced and solutions adopted when seeking to use antitrust law to address market power problems arising in systems of healthcare provision. In the United States, where market solutions have greatest acceptance, antitrust has played and continues to play an important role in setting boundaries regarding conduct that providers and payers may undertake. In European jurisdictions, despite the apparent absence of markets, antitrust has played a similar role on the provider side while leaving the payer side largely untouched. Thus, antitrust has been used to prevent the use or abuse of market power held by healthcare professionals, institutional healthcare providers, or a combination of the two groups, to ensure that such agreements are in the interests of healthcare service users rather than the professionals or providers themselves. Where antitrust has struggled in European jurisdictions is with the behavior of payers and the decisions that payers make—revealing, in Europe at least, some limits of antitrust.