The mammary gland is a complex organ consisting of multiple cell types that undergo extensive remodeling during pregnancy and involution, cyclical changes that suggest the existence of a resident stem cell population that is responsible for remarkable tissue regeneration. The basic functional unit of the mammary gland is the terminal duct lobular unit, which invades the stromal tissue (fat, connective tissue, blood vessels, etc.). Luminal epithelial cells line the ducts while outer myoepithelial cells secrete the basal lamina that separates the mammary gland parenchyma from the mesenchymal cells of the stroma. Within the epithelial cell population of the ducts resides the mammary gland stem cells and it is believed that this population is the origin of the mammary gland cancer stem cells as well. In the mouse, epithelial stem cells can be separated from mesenchymal cells on the basis of CD24, CD44, and CD49f expression. This allows for the determination of both normal and cancer stem cell potential of these two populations and permits investigation into their interaction in tumor development.