This article examines factors responsible for the stark racial disparities in HIV infection in the U.S. and the now concentrated epidemic among African Americans. Sexual network patterns characterized by concurrency and mixing among different subpopulations, together with high rates of other sexually transmitted infections, facilitate dissemination of HIV among African Americans. The social and economic environment in which many African Americans live shapes sexual network patterns and increases personal infection risk almost independently of personal behavior. The African-American HIV epidemic constitutes a national crisis whose successful resolution will require modifying the social and economic systems, structures, and processes that facilitate HIV transmission in this population.