Mycoplasma synoviae belongs to the class Mollicutes which includes the mycoplasmas, the ureaplasmas, the acholeplasmas and the spiroplasmas. These organisms, most of which are parasites or commensals of plants, insects and animals, are among the smallest prokaryotes capable of independent life. Numerous factors have been associated with the pathogenicity of Mycoplasma infection, including cytadhesion to host target cells, evasion of host immune defences, stimulation of host immune-related self injury, toxin production and harmful effects on host cell metabolism. In animals, mycoplasmas usually colonise mucosal epithelial surfaces of the upper respiratory and urogenital tracts and the mammary gland; however, some mycoplasmas have the potential to invade systemically. Strains of M. synoviae differ in the severity with which they cause disease in poultry, from subclinical infection to upper respiratory disease and, occasionally, septicaemia and synovitis. Current findings indicate that the pathogenicity of M. synoviae in poultry involves attachment and colonisation of the organisms in target tissues; however, one or more as yet unidentified factors seem also to be necessary.