The Chlamydiaceae are obligate intracellular bacteria that produce a wide variety of infections in many mammalian and avian species. Three species belonging to two genera of Chlamydiaceae infect humans: Chlamydia trachomatis, Chlamydophila psittaci, and Chlamydophila pneumoniae. C. trachomatis is exclusively a human pathogen and is transmitted from person to person via sexual contact, perinatal transmission, or close contact in households. C. psittaci, in contrast, is more widely distributed in nature, producing genital, conjunctival, intestinal, or respiratory infections in many avian and mammalian species. C. pneumoniae is a fastidious organism that produces upper respiratory tract infection and pneumonitis in both children and adults. This chapter details the epidemiology, pathogenesis, diagnosis, and treatment of chlamydial diseases. Sexually transmitted diseases, perinatal infections, adult inclusion conjunctivitis, trachoma, and psittacosis are covered. The chapter also includes tables outlining comparative features of the three species and treatment of sexually transmitted diseases caused by C. trachomatis, as well as a figure illustrating the life cycle of Chlamydiaceae. This review contains 1 highly rendered figure, 2 tables, and 87 references.