Chlamydia trachomatis genital infections are among the most common sexually transmitted diseases in the United States today. Although these organisms are obligate intracellular pathogens, they more closely resemble bacteria than viruses. C. trachomatis is responsible for considerable morbidity in women, causing urethritis, cervicitis, endometritis, and pelvic inflammatory disease. The latter complication is associated with a high incidence of infertility and ectopic pregnancy, even when the infection is asymptomatic. In young men, C. trachomatis is a common cause of urethritis and epididymitis. Diagnostic tests include tissue culture which has the greatest sensitivity and specificity but is difficult and costly, and various antigen assays which are useful in high-risk, high-prevalence populations. Treatment is effective with doxycycline or erythromycin, but success also depends on appropriate follow-up and empiric treatment of sexual partners. Control of C. trachomatis genital infections is crucial to the control of all sexually transmitted diseases including HIV infection.