Mammary gland development and function would not be possible without tissue-specific stem cells. The mammalian reproductive cycles of pregnancy-associated proliferation, lactational differentiation, apoptosis and remodelling following weaning may occur many times during a female's reproductive years. Such processes necessitate a population of tissue-specific stem cells that have a near unlimited capacity to generate the short-lived, differentiated breast cells. In contrast to the functional cells, breast stem cells must last throughout the life of an organism. Because of this longevity, stem cells may accumulate genetic alterations that eventually lead to cancer. Breast tumors contain a population with stem-cell characteristics. Current tumour therapy modalities target proliferative cells, and be successful in causing tumor regression. Targeting these tumor stem cells will be an important goal of future research.