This article addresses the epidemiology of several common sexually transmitted infections (STIs) in adolescents. Chlamydia is a common occurrence in adolescents, more so than is gonorrhea, but both are of particular concern because they may cause pelvic inflammatory disease. Many experts recommend screening for chlamydia in sexually active adolescents, particularly females. Trichomonas vaginalis is significant as a marker for risk of contracting other STIs and because of its association with pregnancy complications and with increased risk of transmission of HIV. Genital herpes simplex virus (HSV) infection, which usually has been caused by HSV-2, is a common finding in adolescents, and it now is caused also by HSV-1 in some populations. Human papillomavirus (HPV), though widespread in adolescents, usually is a self-limited infection, and malignancy resulting from HPV is a rare occurrence in this age group. The least common of the diseases discussed below is syphilis, but a recent sharp increase in incidence has occurred in men who have sex with men.