This article presents the logic, methods, and capabilities of a major new source of data on the Medicare population, the Medicare Current Beneficiary Survey (MCBS). The survey originated from the need to provide valid estimates of various kinds of health care spending, such as long-term care spending or expenditures by different age groups, to describe the effects of the Medicare program on its beneficiaries, and to model the effects of proposed program changes. Presented here is an account of the MCBS sampling and data collection design and the analytic strengths of the resulting data. Of special interest are the use of Computer-Assisted Personal Interviewing (CAPI); sampling from Medicare enrollment files; design for both cross-sectional and longitudinal analysis; surveying both community and facility residents; and merging survey and administrative data.