This brief is a primer that explains how federal housing assistance programs work and how they, and the problems they are designed to address, have changed over the past decade. The data show that renter housing needs have grown substantially – almost totally because of unaffordably high rents rather than physical deficiencies – and that federal housing assistance is far from keeping up. The number of low income renters that actually receive federal housing assistance has dropped notably as a fraction of the low income households that need it. From the evidence available now, it seems almost certain that this gap will continue to worsen over the next several years.