In 2012, breast cancer was the second most commonly diagnosed cancer worldwide with an approximate 1.7 million diagnoses (11.9 percent of new cancer diagnoses). Breast cancer is the result of a deregulated biological process in which normal breast cells have been transformed and are able to self-renew indefinitely. The application of stem cells to breast cancer is twofold. First, the study of stem cells has led to greater understanding of cancer cell biology and the concept of the cancer stem cell (CSC). Second, stem cell research has led to insights into the development of breast tissue and the molecular mechanisms leading to breast cancer, which has provided new targets and potential treatment modalities for breast cancer.