The United States government devotes about $40 billion each year to means-tested housing programs, plus another $6 billion or so in tax expenditures on the Low Income Housing Tax Credit (LIHTC). What exactly do we spend this money on, why, and what does it accomplish? The authors focus on these questions. They begin by reviewing the history of low-income housing programs in the U.S., and then summarize the characteristics of participants in means-tested housing programs and how programs have changed over time. The authors consider important conceptual issues surrounding the design of and rationale for means-tested housing programs in the U.S. and review existing empirical evidence, which is limited in important ways. Finally, we conclude with thoughts about the most pressing questions that might be addressed in future research in this area.