This paper makes the argument that a public plan is important to health reform because it will contribute to cost containment, primarily by addressing problems caused by increased concentration in insurance and hospital markets. We describe how the public plan might be structured, how many people might be expected to enroll, and how much money the public plan might save. We discuss the most frequent arguments that are made in opposition to the public plan. We conclude that the private insurance industry would survive at about the same size but be more efficient and more effective in controlling health care spending.