**Abstract**

Bacteria of the genus *Chlamydia* belong to the order *Chlamydiales*, within the family *Chlamydiaceae*. These intracellular parasites have a different biphasic reproductive cycle than other bacteria. The important *Chlamydiaceae* are *Chlamydia trachomatis*, *Chlamydophila pneumoniae* and *Chlamydophila psittaci. Chlamydia trachomatis* and *Clamydophila pneumoniae* are primary human pathogens. *Chlamydia trachomatis* is transmitted by sexual contact. It is the causative agent of LGV (lymphogranuloma venoreum) and ocular trachoma in humans. *Chlamydophila pneumoniae* causes bronchitis, atypical pneumonia, sinusitis, pharyngitis, and inflammatory atherosclerosis. *Chlamydia psittaci* is the causative agent of psittacosis (pneumonia). It primarily causes infection in birds and domestic animals, and sometimes in humans. *Chlamydia trachomatis* laboratory diagnosis is based on cytological examination (Giemsa), antigen detection (with enzyme-linked immunosorbent assay and direct immunofluorescence staining), nucleic acid-based tests (nucleic acid probe tests and nucleic acid amplification tests—NAAT), cell culture (*in vivo* and *in vitro*), and detection of antibodies (especially microimmunofluorescence—MIF and enzyme immunoassay— EIA, for the diagnosis of LGV). The most specific test in diagnosis is cell culture, and the most sensitive is nucleic acid-based test. NAAT and MIF tests are successful in the diagnosis of *C. pneumoniae* infections. The diagnosis of psittacosis is usually made by serological testing, and species-specific MIF testing should be performed to confirm.

**Keywords:** *C. trachomatis*, *C. psittaci*, *C. pneumoniae*, Labaratory diagnosis, chlamydia infections
