The concept of mammary epithelial stem cell is tightly linked to the unsolved question of tumor initiation, treatment response and course of disease. Progress regarding the molecular and phenotypical characterization of specific cellular compartments in different periods of life within the human female breast, as well as the understanding of their biological properties has led to the identification of immature stem-/progenitor cells. Since there is no uniform method of identification of these cells universal agreement exists only on their definition based on their biological properties. The concept of a mammalian epithelial stem cell explains several clinical presentations of breast cancer disease (e.g. hereditary breast cancer). However, therapeutic application is currently in the early stages of clinical trials.