Prolactin is a tetrahelical cytokine most closely related to growth hormone and placental lactogens. It binds to specific prolactin receptors that belong to the WS-motif cytokine receptor family. Prolactin is secreted in a highly regulated manner into the circulation by the anterior pituitary, and acts on peripheral target tissues as a hormone. In addition, prolactin is expressed at many extrapituitary sites, particularly within the female and male reproductive organs and the cells of the immune system, acting locally as an autocrine or paracrine cytokine. Because of the ubiquitous expression of prolactin receptors, prolactin has a wide range of cellular and physiological effects. In mammals, prolactin is particularly critical for the differentiation of the mammary gland and for lactation. Hyper prolactinemia, the most common pituitary disorder, causes infertility and decreased libido in both men and women. Prolactin may also influence the progression of certain autoimmune diseases, and has been implicated as a promoter of neoplastic growth.