Mycoplasma hominis was isolated from a Bartholin's gland abscess 70 years ago, and ureaplasmas were isolated from the genital tract about 20 years later. Subsequently, reports incriminating mycoplasmas in the known adverse outcomes of pregnancy have been legion. Without doubt these genital mycoplasmas are able to invoke an inflammatory response and take part in the cascade of events that culminates in preterm birth. Their role in this and other conditions is becoming clearer, but controversy remains due mainly to investigators often ignoring bacterial vaginosis, with its complex of bacteria, and failing to disentangle their role from that of the genital mycoplasmas. This is a theme that will be highlighted in this chapter, in which an attempt is made to indicate what is indisputable (surprisingly little) and what is not, and where further research would be helpful.