Chlamydiae are obligate intracellular parasites with a unique biphasic life cycle. The genus Chlamydia is classified in the family Chlamydiaceae of the order Chlamydiales. Within this genus four species are recognized currently: C. pecorum, C. psittaci, C. trachomatis, and C. pneumoniae. All except C. pecorum have been associated with human disease. C. trachomatis has been most extensively studied because of its association with ocular trachoma and its prevalence as a sexually transmitted pathogen. C. psittaci is common in birds and domestic animals. Psittacosis is a systemic infection that frequently causes pneumonia. C.pneumoniae is a significant cause of pneumonia in both the hospital and outpatient settings. C. pneumoniae has been associated with atherosclerotic cardiovascular disease by the results of seroepidemiologic studies, by detection of the organism in aterosclerotic plaque specimens, by experimental in-vitro cell culture studies, by animal model studies.