I. Introduction THE mammary gland is a unique feature of mammals. Its chief biological function is to synthesizeand secrete milk, which is essential for the nourishment of mammalian young. The parenchymal portion of the mammary gland is derived embryologically from ectoderm, while the surrounding stroma comes from mesoderm. Mammary gland growth takes place during both fetal and postnatal life; the gland is the site of milk production and secretion, and in several species (including humans) it is also a site of occurrence of preneoplastic and neoplastic lesions. Thus the mammary gland is an important organ of study by biologists from various disciplines, including endocrinologists, tumor biologists, and developmental biologists. During the last 30 yr, the primary interest of our laboratory has been the understanding of the neoplastic transformation process in mammary epithelial cells. One fundamental characteristic of tumor cells vis à vis their normal counterparts is deregulation of growth. Since cell proliferation is a prerequisite for neoplastic transformation in target tissues, we have tried to understand the regulation of mammary epithelial cell growth. Toward this end we have utilized a variety of in vivo techniques, in vitro organ and explant culture systems, and also cell culture methodologies. These tools have aided in the analysis of the roles of hormones, growth factors, lipids, metabolic products, and even extracellular matrix components in the control of growth in mouse and rat mammary epithelial cells.