Human prolactin (PRL) is currently viewed as a hormone of pituitary origin, whose production (i.e. serum levels) is controlled by dopamine, whose biological actions relate exclusively to lactation and reproductive functions, for which any genetic disorder is yet to be identified, and whose unique associated pathology is hyperprolactinemia. Both experimental studies and human sample/cohort-based investigations performed during the past decade have considerably widened our perception of PRL biology: i) there are now strong epidemiological arguments supporting the fact that circulating PRL is a risk factor for breast cancer, ii) in addition to the endocrine hormone, locally produced PRL has been documented in several human tissues; there is increasing evidence supporting the tumor growth potency of local PRL, acting via autocrine/paracrine mechanisms, in both rodent models, and human breast and prostate tumors, iii) the first functional germinal polymorphisms of the PRL receptor were recently identified in patients presenting with breast tumors, which involve single amino acid substitution variants exhibiting constitutive activity, iv) human PRL analogs have been engineered, which were shown in experimental models to down-regulate the effects triggered by local PRL (competitive antagonism) or by the constitutively active receptor variants (inverse agonism). The aim of this review is to discuss these novel concepts in PRL biology, including their potential pathophysiological outcomes.