Sexually transmitted diseases (STDs) are a major public health problem, affecting predominantly adolescents. The past decade has witnessed major developments in the epidemiology of STDs. “Traditional” STDs, such as syphilis and gonorrhea, are increasingly concentrated in high prevalence or “core” areas, which are typically in the inner city communities with high rates of drug use and other public health problems. Sexual behavior associated with crack cocaine has had a substantial impact on the epidemiology of STDs, especially syphilis. Because drug use and STDs are both independent risk factors for human immunodeficiency virus infection, human immunodeficiency virus counseling and testing has become an integral part of STD control programs. The high prevalence of antibiotic-resistant gonococcal infections has resulted in new public health needs for surveillance and has made penicillin obsolete for therapy. Chlamydial control programs are hampered by a relative lack of funds and by the high proportion of disease that occurs without signs or symptoms. Defining the epidemiology of chronic viral STDs, such as herpes simplex and human papillomavirus infection, is difficult. These diseases are widely prevalent and overwhelmingly symptomatic, and diagnostic tests for chronic herpes simplex virus and human papillomavirus suffer from both sensitivity and specificity problems.