Chlamydiales, one of the oldest bacterial orders in evolutionary terms, are widespread among animals. Blinding trachoma, a disease caused by Chlamydia trachomatis, was already known in ancient times, whereas modern reports on psittacosis date from 1879. Though these pathogens have long been known and lead to serious health problems both in human and animals, data on Chlamydiales biology has been limited. It is due to their intracellular life style and complex developmental cycle. New molecular biological methods have been recently developed expanding the possibilities of chlamydial research and diagnosis. This paper reviews data concerning avian chlamydiosis, its aetiological agent C. psittaci, newly proposed species isolated from birds, namely C. ibidis sp. nov., C. avium sp. nov., and C. gallinacea sp. nov., and their zoonotic potential.