The mouse mammary gland is a complex tissue that proliferates and differentiates under the control of systemic hormones during puberty, pregnancy and lactation. Once a highly branched milk duct system has been established, during mid/late pregnancy, alveoli, little saccular outpouchings, sprout all over the ductal system and differentiate to become the sites of milk secretion. Here, we review the emerging network of the signaling pathways that connects hormonal stimuli with locally produced signaling molecules and the components of intracellular pathways that regulate alveologenesis and lactation. The powerful tools of mouse genetics have been instrumental in uncovering many of the signaling components involved in controlling alveolar and lactogenic differentiation.