The US. health care environment is changing rapidly. As employers and public agencies have tried to control spending, more health care is provided through managed care organizations. This approach to delivering care has reduced hospital use and led to thousands of empty beds. It also has spurred provider collaboration, for example through hospital mergers, physician networks, and physician-hospital delivery systems. Competitive pressures have also encouraged health insurers and HMOs to merge into large national or regional plans. An important issue facing health policy makers who are relying on competition to reduce health care spending is how this consolidation affects consumers. Does it reduce costs through increased efficiency and improve services through coordination? Or does it create organizations with market dominance that lead to higher prices and lower quality? This report examines the implications for health care cost, quality, innovation, and choice of the trends toward health care market consolidation.