Aerobic actinomycetes causing nocardiosis, GRT-(Gordona, Rhodococcus, Tsukamurella) infections, actinomycetomas, dermatophilosis and extrinsic allergic alveolitis, including etiology, epidemiology and pathogenesis of the diseases are described. The name "actinomycetosis" for all these diseases has been proposed. Many clinical important and potential pathogens, as well as an increasing number of opportunistic, immunosuppressed and immunocompromised cases are discussed. A review of the abundant literature up to date, concerning the problem, is presented.