Avian chlamydiosis (AC) is caused by infection with a Chlamydia species in birds. In 2015, the taxonomy of the family Chlamydiaceae was revisited. The genus Chlamydia currently includes 11 recognised species, and among them C. psittaci, C. avium and C. gallinacea have been isolated from birds. Outbreaks of AC in psittacine birds and domestic poultry farms cause considerable economic damage. The infection can lead to systemic and occasionally fatal disease in birds. The clinical signs are generally nonspecific and vary greatly in severity, depending on the species and age of the bird and the virulence of the Chlamydia strain, but respiratory distress is mostly involved. Many birds, especially older psittacine birds and poultry, may show no clinical signs; nevertheless, they may often shed the agent for extended periods. There are no commercial vaccines available for chlamydiosis control in poultry. Avian chlamydial strains can cause serious illness (pneumonia) and death in humans when left untreated.