Trichomonas vaginalis is a sexually transmitted surface pathogen of the lower urogenital tract, and may be associated with asymptomatic vaginal colonization or intensely symptomatic vaginitis. In pregnancy it is associated with an increased risk of preterm delivery. However, a randomized trial of treatment of asymptomatic trichomonas colonization in pregnancy showed an increase in the risk of preterm delivery in treated women. The reasons for this paradox are yet to be fully elucidated. Candida species, on the other hand, may be present--usually in the yeast form--in the vaginal flora of up to 40% of healthy pregnant women. Although candidiasis is not usually associated with chorioamnionitis or preterm delivery, there is some emerging evidence to suggest that screening for and eradication of candida during pregnancy may reduce the risk of preterm delivery. This chapter reviews the impact of these common vaginal infections on pregnancy outcome and appraises the recent evidence on the role of treatment during pregnancy.