One of lawmakers' highest health-related priorities is adding a prescription drug benefit to Medicare. Although that program gives older Americans broad insurance coverage for many health needs, it provides only limited coverage of drugs not dispensed during a hospital stay. That gap in coverage has become increasingly significant as prescription drugs have assumed greater importance in the treatment of disease and as spending for outpatient prescription drugs has soared. Designing a Medicare drug benefit is a complex task, however. The competing goals for such a benefit mean that policymakers must make trade-offs (such as between broad coverage or widespread enrollment and cost). They must consider many different design elements and how those elements might interact. And they must try to avoid various problems that arise in creating such a benefit. Ultimately, the choices that designers make will affect not only the cost of the benefit but a host of other factors, such as demand for and prices of prescription drugs, spending by other federal and state programs, and how various parts of the market for health insurance operate. The Congressional Budget Office (CBO) has had to wrestle with those issues in recent years to produce cost estimates of the various Medicare drug proposals debated in the Congress. This study summarizes the main design choices facing policymakers and explores the implications of those choices for cost and coverage. To illustrate the effect of those choices on cost, it also discusses CBO's cost estimates for four proposals that represent a broad array of designs for a Medicare drug benefit. For the sake of simplicity, this study focuses on a standalone drug benefit-one that would be a freestanding addition to Medicare rather than part of a broad effort to restructure the program. Broad reform proposals raise important policy issues but ones that are beyond the scope of this analysis.