The SSI program provides cash assistance to some of the nation’s most vulnerable elderly, blind, and disabled residents. In this paper, we briefly summarize the history of the SSI program and present descriptive evidence on caseload composition and trends. We discuss relevant conceptual issues and empirical evidence focused on four key issues. First, we describe the advantages and disadvantages of categorical eligibility requirements and we show that the SSI caseload has become increasingly comprised of difficult-to-verify conditions, namely pain and mental disabilities.