Chlamydia trachomatis was first described by Halberstadter and Von Prowazek in 1907, as a characteristic intracytoplasmic inclusion in the conjunctival scrapings from a patient with trachoma. Although the organism was identified as the etiologic agent of trachoma and inclusion conjunctivitis in the neonate, a full understanding of its nature was not feasible until C trachomatis was first successfully propagated in cell cultune in 1965. This technical innovation greatly facilitated investigation of the biology and the epidemiology of chlamydial infections in man. C trachomatis is now recognized to be one of the most prevalent sexually transmitted diseases in the United States today. It is responsible for 40% to 50% of cases of nongonococcal urethritis (NGU) in men and a large reservoir of genital infection, largely asymptomatic, in women. In the United Kingdom, where NGU is a reportable disease, NGU is now more prevalent than gonorrhea and is considered to be the most important male veneral disease. The cervical infection in women is of special importance to pediatricians as the infection may be transmitted to the infant during parturition, leading to both ocular and respiratory disease. A thorough understanding of the microbiology and epidemiology of C trachomatis is an important prerequisite for appreciating the clinical features, diagnostic methods, and management of these infections by pediatricians.