This book explores the theory behind the epidemiology of sexually transmitted infections (STIs) based on the natural history of infections. A wide variety of STIs are covered, including gonorrhea, syphilis, chlamydia, chancroid, trichomoniasis, herpes simplex virus, human papilloma virus, and human immunodeficiency virus (HIV). It describes the advances in measuring patterns of sexual contacts within populations as well as the observed relationships between STIs and risk factors at the individual and population levels. It then reviews the available behavioral and biomedical interventions against STIs and the evidence for their efficacy, effectiveness, and impact. These issues are examined using mathematical models derived from population-based surveys of sexual behavior and STIs. It is therefore unique in drawing together theoretical and empirical research on STI epidemiology in a coherent framework.