Human Mammary epithelial cells (HMEpiC) lie within the alveolar lumen of the breast lobules; which together with mammary ducts and adipose tissues, form a complex network in the mammary gland. MEpiC respond to various growth factors and hormonal cues and undergo changes in several processes; such as growth, invasion, and differentiation during pre- and postnatal stages, puberty, and pregnancy. Aberrant levels of hormones and extracellular matrix composition, and other genetic factors induce uncontrolled proliferation of MEpiC. This process results in breast cancer development. Understanding the cellular properties of MEpiC will help identify the disease mechanisms in breast cancer and the new targets for therapeutic development.