The federal requirement for most Americans to have health insurance-the individual mandate-is an important part of how the ACA would reduce the number of uninsured. We use the Health Insurance Policy Simulation Model to estimate the effects of health reform with and without the mandate. With the mandate, the number of uninsured would decrease from 50 million to 26 million. Without a mandate, about 40 million would remain uninsured. Depending on the effectiveness of the health benefit exchanges in enrolling those eligible for subsidized coverage, exchange premiums would be 10 to 25 percent higher without a mandate.