Chlamydiae are a heterogeneous group of strict intracellular bacteria. Chlamydia trachomatis is the most common cause of sexually transmitted bacterial infection. Being generally asymptomatic, C. trachomatis infections are a major cause of tubar infertility and extra-uterine pregnancy. C. pneumoniae may cause asthmatic bronchitis. C. psittaci infections, although usually developped after exposure to infected birds, has recently been documented following exposure to infected horses. Waddlia chondrophila is an emerging human pathogen of miscarriage. Parachlamydia acanthamoebae, Protochlamydia naeglerophila and to a lesser extent Simkania negevensis, have been associated with lower respiratory tract infections.