Avian mycoplasmosis is caused by several pathogenic mycoplasmas among which Mycoplasma gallisepticum (MG) and M. synoviae (MS) are considered the most important. MG and MS belong to the class Mollicutes, order Mycoplasmatales, family Mycoplasmataceae. MG causes chronic respiratory disease of domestic poultry, especially when flocks are stressed or other respiratory pathogens are present. The disease is characterised by coryza, conjunctivitis, sneezing, and sinusitis, particularly in turkeys and game birds. It can also cause upper respiratory disease in game birds. In poultry the infection is spread vertically through infected eggs and horizontally by close contact. MG can result in significant production losses and downgrading of meat-type birds, and loss of egg production. Although the preferred method of control is maintenance of MG- and MS-free flocks, both live and inactivated vaccines are used in chickens. Vaccination should be considered only in specific cases on the basis of the epidemiological situation in the area or in farms where infection is inevitable. There have been no reports of MG or MS infection in humans.