US healthcare markets have consolidated dramatically over the past several decades to their highest point in history. Federal and state antitrust law, although designed to guard against anticompetitive agreements and behaviors by private market actors that harm competition and consumers, has largely failed to protect healthcare markets and consumers, resulting in higher prices, marginal to poorer quality outcomes, and reduced patient access. Unlike early healthcare consolidation that occurred largely through traditional horizontal transactions, such as the merger of two local hospitals, modern transactions are often much more complex, involving non-horizontal elements, health systems with a wide range of facilities and provider types, out-of-industry entities (e.g., private equity and technology companies), and providers in multiple geographic regions. Antitrust law has displayed little ability to prevent, constrain, or contain the consolidation and market harms arising from these modern transactions. As a result, US healthcare prices, the highest in the world by a wide margin, continue to rise, while employee wages have stagnated, and businesses and government entities continue to falter under the weight of healthcare expenses. All is not lost, however. A combination of private antitrust law that reflects the realities of modern healthcare, aggressive enforcement, and market regulation can begin to turn the tide. This chapter will examine the current state of healthcare markets in the United States, how they got there, and the tools available to federal and state antitrust enforcers and policymakers to address the harms arising from healthcare consolidation, including merger review, conditional approvals and consent decrees, antitrust litigation, legislation, and provider payment regulation.