Mycoplasmas is the trivial name for a complex and unique group of bacteria which belong to the class Mollicutes (soft skin) and have been arranged into 9 genera (Acholeplasma, Anaeroplasma, Asteroleplasma, Entomplasma, Mesoplasma, Mycoplasma, Spiroplasma, Ureaplasma, Candidatus Phytoplasma). More than 200 mycoplasma species have been reported, with the genus Mycoplasma being the largest group with more than 100 described species. Mycoplasmas are characterised by their permanent lack of a cell wall, a small genome size (0.58-2.2 Mbp) and a low G+C content (23-40 mol%) in their genomes. A number of Mycoplasma and Ureaplasma species are human and animal pathogens, causing a variety of diseases predominantly affecting the respiratory tract, the urogenital tract and the joints. The role of mycoplasmas as infectious agents has previously been largely ignored because of the widespread, but misguided clinical perception that these microorganisms are unimportant and that rapid diagnostic procedures are not available. This situation is now changing because of the greater appreciation for the clinical importance of these organisms as pathogens and improved methods for their detection.