Approximately 5 per cent of dairy cows are affected by retained fetal membranes. Retained fetal membranes are an important risk factor for the metritis/endometritis disease complex occurring in early lactation, and costs the UK dairy industry approximately £16 million annually in lost production. Veterinary clinicians have described the clinical signs, treatment and some associated risk factors of the condition since early Victorian times, and these have not changed over time. Research carried out within the past 20 years suggests that there is an immunological basis for retained fetal membranes. In a normal calving, the maturation of the placenta and its separation from healthy caruncles depends upon incompatibility between maternal and fetal major histocompatibility complex class I expressed on epithelium within the fetomaternal unit. Placental maturation follows stimulation of the maternal immune response and the production of neutrophilactivating factors within the epithelium in the caruncular arcade. This affects the extracellular matrix components within the placentome, breaking down collagen within the chorionic villi and assisting in separation from the caruncle. Factors influencing normal placental maturation include downregulation of antioxidative defence mechanisms against reactive oxygen species, a lower ratio of prostaglandin E2 to prostaglandin F2α within the fetomaternal compartments of the placentome, and an elevated steroid hormone receptor status, which reduces the rate of apoptosis occurring in the chorionic epithelium before calving.