Mycoplasmas are a class of fascinating wall-less bacteria that cause disease in humans, animals, and plants. They include species recognized as among the smallest known free-living cells. These microbes have very small genomes and unique genetic features. Their temporary classification as viruses and fastidious growth requirements set the stage early for the use of diagnostic techniques better suited to viral detection and for the well-known difficulties of diagnosing infections in the clinical laboratory. Systemic infections are well known in both immunocompetent and immunocompromised patients, but the host and bacterial factors that allow invasion from epithelial colonization sites are poorly understood. The focus of this chapter will be respiratory and extrapulmonary infections caused by Mycoplasma pneumoniae and genitourinary tract infections caused by M. hominis, M. genitalium, and Ureaplasma species. Some less common mycoplasma infections will also be addressed. © Springer Science+Business Media, LLC 2009.