This paper examines the history, rules, and economic implications of the Medicaid program. I begin by providing a detailed overview of how the program works. I then provide information on who is covered, who is eligible, and spending patterns. I then turn to a review of the economic issues involved in studying the Medicaid program: assessing the impacts on insurance coverage (public and private), health, labor supply, family structure, and savings. I follow this with a review of the empirical literature on each of these topics. Finally, I conclude with a discussion of the policy issues and unanswered questions surrounding the Medicaid program.