Mycoplasma gallisepticum is the most economically significant mycoplasma pathogen of poultry, and has a world-wide distribution. In common with other mycoplasmas, M. gallisepticum is minute in size with minimal genetic information and with a total lack of a bacterial cell wall. These properties are reflected in a high degree of interdependence between M. gallisepticum and the host animal, and in the fastidious nature of the organism in vitro. Strains of M. gallisepticum differ markedly with respect to important biological properties such as pathogenicity, infectivity, tissue tropism and transmissibility. In addition, phenotypic variation of major surface antigens occurs at high frequency, which is a probable explanation for chronic infection by M. gallisepticum despite a strong immune response. Infection with M. gallisepticum has a wide variety of clinical manifestations, but even in the absence of overt clinical signs, the economic impact may be significant. The most dramatic disease presentation of M. gallisepticum is chronic respiratory disease in meat-type birds, often as one of several aetiological agents in a multi-factorial disease complex. Transmission of M. gallisepticum in ovo from infected breeder birds to progeny is the major route of dissemination of the infection, and is the prime consideration for international trade. In most countries, control programmes for M. gallisepticum are based on maintaining commercial breeding stock free of infection. In instances where control of M. gallisepticum infection is not feasible, vaccination, especially with newly developed live M. gallisepticum vaccines, is being evaluated as an option. Major advances in diagnostic methods have been made in recent years. Control programmes have been based on serological methods, with screening for infection usually accomplished by the slide plate agglutination (SPA) test or by enzyme-linked immunosorbent assay. Further serological testing and/or demonstration of the presence of the organism must be used to confirm SPA suspected positive tests. In principle, detection of the presence of the M. gallisepticum organism can be by isolation of the organism or detection of the deoxyribonucleic acid by molecular methods. Polymerase chain reaction represents a rapid and sensitive alternative to traditional culture methods, which require time-consuming specialised techniques. The development of molecular typing methods affords new opportunities for epidemiological studies and identification of reservoirs of infection.