Mammary gland is a modified sweat gland evolved exclusively to nourish the young. In cows, four mammary glands are joined together to form an udder from which the milk is drained through teat. Internally the mammary gland is lobuloalveolar in nature composed of secretory tissues, connective tissues and ductular system. The secretory tissue produces the milk and drains through the ductular system. Connective tissues act as a supporting system to the udder. The primary secretory unit of the mammary gland is the alveoli lined by luminal epithelial cells and myoepithelial cells. Luminal epithelial cells produce the milk, and contractile myoepithelial cells support the gland and help in milk secretion. These secretory tissue components are embedded within the stromal tissue comprising fibroblasts, adipocytes, extracellular matrix proteins, blood vessels, and lymphatics. The contact between epithelial cells and myoepithelial cells is facilitated by tight junctions, adherens junctions, desmosomes, and gap junctions, and the cellular interactions are mediated by junctional proteins such as zonula occluden, occludin, cingulin, cadherin, catenin, desmoglein, desmocollin, and connexin. The interaction between mammary epithelial cells and ECM is crucial for the development and survival of mammary epithelial cells. These interactions are facilitated by transmembrane receptors (integrins) in the mammary epithelial cells and their ligands (collagen, laminin, fibronectin, vitronectin) in the ECM components.