Mycoplasma gallisepticum, Mycoplasma synoviae and Mycoplasma meleagridis are the most widespread pathogenic poultry mycoplasmas. The epidemiology of the infections, and clinical signs, lesions, pathogenesis, diagnosis and control of disease are described. In epidemiology, the constant antigenic composition, variation in virulence, tropism, survival of the mycoplasma outside and within the host and susceptibility to drugs are outlined. The significance of species, age and immune status of the host and the importance of intercurrent infection and debilitating factors in the production of disease are given. The transmission of infection, clinical signs of disease, and gross and histopathological appearances of lesions are described. The interpretation of the pathogenesis of disease is based on the appearance of lesions in untreated birds and in bursectomized or thymectomized chickens. The means of recognizing infection, and the methods used in ameliorating the worst effects of disease or eradicating infection follow.