With the growing elderly population, we are aware of the increasing resource needs for medical care and anticipate an increasing burden on social welfare system. Medicaid and Medicare are at the edge of financing crisis at both the federal and state level. This book examines effects of Medicaid and Medicare on medical care use for U.S. elderly from an academic perspective of economics. Medicaid studies identify elderly eligible for Medicaid who consist of two groups ¿ the targeted pre-existing poor and the non-targeted elderly who may spend down to be eligible. Exploring Medicaid participation determinants and spend-down behaviors, we estimate effects of Medicaid on medical services use. Medicare study tracks adults for a period of ten years, that is four years prior to Medicare and six years afterward. Identifying individuals¿ private health insurance status prior to Medicare as previously insured and uninsured, we compare their medical expenses and health outcomes biannually before and after coming into Medicare. The studies should shed some light on how Medicaid and Medicare fare to date, and should be especially useful to researchers and policy-makers in health related areas.