Chlamydiae are among the most successful bacterial pathogens, and there are few branches of medicine on which chlamydial infection and its sequelae do not impinge. Chlamydia trachomatis is responsible for many million cases of blindness, pelvic inflammatory disease, urethritis, epididymitis, infertility and ectopic pregnancy annually; it also causes lymphogranuloma venereum, reactive arthritis, ophthalmia neonatorum and infantile pneumonia. C. pneumoniae is among the most common causes of community-acquired pneumonia, and recent evidence suggests that it may play a part in the pathogenesis of coronary heart disease. C. psittaci is a highly prevalent zoonotic infection with a wide host range. It is of great economic importance, and causes sporadic but sometimes devastating disease in humans. Most chlamydial infections are subclinical, but even if the initial illness is mild there may be serious long-term sequelae. It is therefore important to identify and treat chlamydial infections in their early stages, but diagnosis usually depends on laboratory tests. Recent trials have shown that single doses of the long-acting macrolide azithromycin are effective in the treatment of genital and ocular C. trachomatis infection, but longer courses of antimicrobials remain the mainstay of treatment for C. pneumoniae and C. psittaci infections.