Preterm birth represents a major problem for modern obstetrics due to its increasing frequency and the accompanying socioeconomic impact. Although several maternal characteristics related to preterm birth have been identified, the etiology in most cases remains inadequately understood. Various microorganisms have been linked to the pathogenesis of preterm birth. Microbes may reach the amniotic cavity and fetus by ascending from the vagina and cervix, by hematogenous distribution through the placenta, by migration from the abdominal cavity through the fallopian tubes, or through invasive medical procedures. Organisms commonly cultured from the amniotic cavity following preterm delivery include Ureaplasma urealyticum, Mycoplasma hominis, Bacteroides spp., Gardnerella vaginalis, Neisseria gonorrhoeae, Chlamydia trachomatis, Trichomonas vaginalis, and group B hemolytic streptococci. Several trials have examined the effect of antibiotic administration to patients with preterm labor and intact membranes, preterm premature rupture of the membranes, genital mycoplasmal infection, asymptomatic bacteriuria, and bacterial vaginosis. The results of such studies, which were variable and often conflicting, are discussed here.