It is well established tht the intial interaction of prolactin with its target cells is with specific receptor molecules located on the external surface of the plasma membrane. Subsequent to the prolactin-receptor interaction, however, the molecular events that culminate in the regulation of biological processes are not totally understood. This book was assembled to review and analyse the currently available information relative to the molecular events involved in the actions of prolactin on cells. Hopefully, the assimilation of this information will provide the basis for ultimately determining the sequence of molecular reactions by which prolactin expresses its biological responses.