Mycoplasmas are the smallest free-living organisms, widespread in nature. Several mycoplasma species have been isolated from humans. For 6 of them: Mycoplasma hominis, Ureaplasma urealyticum, M. primatum, M. genitalium, M. spermatophilum and M. penetrans, the genital tract is the main site of colonization. This review is concentrated on the role of mycoplasmas as sexually transmitted agents, with the emphasis to M. genitalium infections. M. hominis and U. urealyticum are isolated from the genital tract of healthy men and women with considerable frequency. The biological features (attachment properties, possible intracellular location) and experimental inoculation studies of M. genitalium indicate that this mycoplasma has pathogenic potential. Data from case-control studies, looking at men with non-gonococcal urethritis and women with cervicitis, have revealed that M. genitalium behave similarly to Chlamydia trachomatis and have revealed that carriage of M. genitalium and C. trachomatis is usually independent of one another. M. genitalium could be considered as a potential cause of sexually transmitted urethritis in men, including men with persistent or recurrent urethritis. More studies are expected to ascertain the role of M. genitalium in the female genital tract. Evidence-based data are needed to decide whether current non-gonococcal infection treatment principles are applicable or not for M. genitalium infections.