Chlamydiae and Chlamydophilae are bacteria which are very common in the environment, the proof of which can be the recently described "environmental Chlamydiae". The classical Chlamydiae, such as Chlamydia trachomatis and Chlamydophila pneumoniae, posed a threat to human health, causing several diseases, including those with lethal course. In this article the biology and systematics of these bacteria and the diseases caused by them in humans, including those caused by classical and environmental Chlamydiae, are presented.