Prolactin is a pleiotropic peptide hormone and cytokine that is secreted from the pituitary gland and locally within various tissues of the body for autocrine and paracrine signal transduction. It controls proliferation and differentiation in a number of body tissues and increasing evidence indicates that it controls these functions in undifferentiated stem and progenitor cells of adult tissues, such as mesenchymal stem cells, hematopoietic progenitors, neural stem cells, oligodendrocyte precursor cells and possibly in mammary gland stem/progenitor cells. These roles in these undifferentiated cell types also implicate prolactin in the stem cell theory of cancer, supporting its known roles in cancer formation and progression.