Concerns about the number of people who lack health insurance and about the high and rising costs of health insurance and health care have led to proposals that would substantially modify the health insurance system in this country. Medicare already provides nearly universal coverage to the elderly, those proposals generally focus on options for providing coverage to and reducing costs for the non-elderly population. They could, for example, provide new federal subsidies to pay some portion of health insurance premiums; impose mandates for individuals to purchase coverage or for employers to offer it and others. This book describes some of the key assumptions that the Congressional Budget Office (CBO) would use in estimating the effects of key elements of such proposals on federal costs, insurance coverage, and other outcomes.