Chlamydia trachomatis infections are highly prevalent worldwide, especially in adolescents and young adults, and can cause substantial morbidity. The majority of persons with chlamydia are asymptomatic with normal genital examination findings, and diagnosis of chlamydia usually relies on testing for the bacteria. Despite highly sensitive tests and highly effective therapy, the number of reported chlamydial infections continues to rise. As a chlamydia vaccine is not available, better prevention and control efforts are needed. C. trachomatis is an obligate intracellular bacterium that infects mucosal surfaces of humans, including oropharyngeal, anogenital, and conjunctival surfaces. C. trachomatis can be classified through molecular typing into strains causing ocular infections (trachoma), strains causing nonulcerative oropharyngeal and/or anogenital infections (chlamydia), and strains causing a distinct ulcerative chlamydia syndrome called lymphogranuloma venereum (LGV). This chapter reviews the epidemiology, clinical features, diagnosis, treatment, and prevention of chlamydia and LGV.