Prolactin is a hormone that is mainly secreted by lactotroph cells of the anterior pituitary gland. It is involved in many biological processes including lactation, metabolism, regulation of the immune system and reproduction. The major isoform acts via its membrane receptor, the prolactin receptor (PRL-R), which is a member of the haematopoietic cytokine superfamily. One of the prolactin isoforms has received particular attention as a result of its recently discovered inhibitory effects on angiogenesis and tumorigenesis. The discovery of multiple extrapituitary sites of prolactin secretion also increases the range of known functions of this hormone. Prolactin and its receptor serve multiple physiological and pathological roles in human pat