Mycoplasmas comprise a big group of organisms consisting of one hundred eighty species which are found in nature as parasites of humans, other mammals, reptiles, fishes and plants, or living as commensals. The group commonly referred to as genital mycoplasmas comprise species most often found in the genitourinary tract of sexually active adults as common commensal inhabitants, or pathogens which can cause many different infections. The species we are most interested in this work are called: Mycoplasma genitalium, Mycoplasma hominis and Ureaplasma urealyticum. There is no doubt that they can cause non-gonococcal urethritis (NGU) in both men and women and bacterial vaginosis (BV), cervicitis, endometritis. Infection can spread to the upper port of female genital tract and it can lead to pelvic inflammatory disease, or if it happens during pregnancy--to chorioamnionitis and further pregnancy complications. Even though mycoplasmas have been known and described since 1898, the problem of their morbidity and the possible influence they have on human fertility is still not clear. Similar to research from 30 years ago, connecting Chlamydia trachomatis with infertility, new scientific work as well as the dynamic development of diagnostics procedure, especially more common use of PCR method, may be helpful in discovering the potential role genital mycoplasmas play in infertility.