The human breast serves the physiologic purpose of production and secretion of milk. Milk that is secreted from the nipple navigates a complex ductal system to reach the nipple-areolar complex. This ductal system begins with microscopic milk-producing acini which are grouped into lobules. The acini have receptors sensitive to serum prolactin levels (highest during the late luteal phase of the menstrual cycle and during pregnancy) and produce milk accordingly. Once produced and secreted at the acinar level, milk travels toward the nipple through a network of intralobular terminal ductules which converge to form ducts. Beneath the nipple-areolar complex these ducts converge to form lactiferous sinuses which then lead to openings in the nipple. The histology of the ductal system is composed of a layer of A cells (milk-producing cells), B (or chief) cells (the energy source), myoepithelial cells, and an underlying basement membrane. Myoepithelial cells contract in response to serum estrogen, progesterone, and prolactin levels, with resultant secretion of milk from A cells. Also, oxytocin is secreted by the posterior pituitary gland in response to nipple stimulation; it results in contraction of myoepithelial cells, thus inducing milk to be secreted from the lobules to the ducts. The entire ductal and lobular network is invested in connective tissue stroma, lymphatics, and adipose tissue. In the postmenopausal breast, tissue parenchyma (ducts and lobules) will involute to variable degree in response to decreased hormonal stimulation.