Mycoplasmas are a bacterial group that is classified in the Mollicute class which includes Mycoplasmas, Spiroplasmas and Acholeplasmas. One hundred and seventy six species have been described in this group. Mycoplasmas are the smallest self living prokaryotes, they do not have a bacterial wall, their genomic size ranges from 577 to 2220 bpk, they are nutritional exigent so it is hard to culture them, but the development of molecular biology techniques has let us detect more mycoplasmas in different hosts. Mycoplasmas have been associated to acute and chronic diseases mainly in animals and humans while spiroplasmas have been found in arthropods, plants and flowers producing or not damage. Some recent studies have shown the role of some structural components of Mycoplasmas in pathogenesis, such as cytoskeleton proteins and adhesins, and the influence of some genetic characteristics on the development of an infectious disease.