The first part of the present review gives an overview on the history of infectious agents of the order Chlamydiales and the general infection biology of Chlamydophila (C.) psittaci, the causative agent of psittacosis. In the second part, the classification of C. psittaci strains, as well as issues of epidemiology of avian chlamydiosis., disease transmission routes, clinical disease, public health significance, present legislation and recommendations for prevention and control are reviewed.