Each sexually transmitted disease (STD) epidemic evolves through predictable phases, shaped by a dynamic interplay among the pathogen, the behaviors of the subpopulations in which it emerges, and the prevention efforts that are developed to limit its impact. As STD epidemics move through these phases, the sexual and social networks that fuel them become located in subpopulations characterized by progressively higher rates of sex partner change and less contact with the health care system. As a result, phase-appropriate prevention strategies and research issues are essential to reducing STDs and their consequences.