Avian mycobacteriosis is an important disease which affects companion, captive exotic, wild and domestic birds. The disease is most commonly caused by Mycobacterium avium and Mycobacterium genavense. Lesions are typically found in the liver and gastrointestinal tract, although many other organ systems can potentially be affected. The authors review those species of Mycobacterium reported to affect birds, the epidemiology of avian mycobacteriosis, immunological responses to mycobacterial infection, ante- and post-mortem diagnosis, treatment and prevention or control of the disease.