Mycoplasmas are prokaryote microorganisms without cellular wall, that usually colonize the respiratory and urogenital mucosae. The pathogenic species for mankind are Mycoplasma pneumoniae, that produce respiratory infections, among them pneumonia, and M. genitalium, M. hominis and Ureaplasma urealyticum, that produce urinary and genital infections. The diagnosis of these infections is fundamentally based on serology, since the culture of these germs is very laborious. In a near future, the antigen or DNA detection molecular techniques will probably permit a much faster and reliable diagnosis. The tetracyclines and fluorquinolones have excellent activity against these microorganisms.