Mycoplasma genitalium is a sexually transmitted disease caused by a small bacterium that colonizes the male and female reproductive tract and was only discovered in the 1980s. The bacteria can evade the adaptive immune system by establishing intracellular infection and can persist for months or years in asymptotic individuals. Infection is associated with acute and chronic urethritis in men and urethritis, cervicitis, pelvic inflammatory disease, and possibly infertility in women. Diagnosis is primarily by molecular testing; however, serology testing is used for epidemiological studies. M. genitalium shares several structural properties with M. pneumoniae, and cross-reactivity between these species can impact serology tests. Consequently, diagnosis is primarily by molecular testing; however, serology testing is used for epidemiological studies.