<p>Tissue specific stem cells are critical in maintaining organ function throughout life. Understanding the dynamics and heterogeneity of stem cells and progenitors is essential to understanding how diseased cell states such as cancer evolve. Studies in the mammary gland have revealed significant heterogeneity in stem cell identity during embryonic development, puberty, adulthood and aging. In this review, we discuss the dynamics of lineage commitment in the mammary gland and the current understanding of mammary stem cells versus lineage committed progenitors at different stages. As breast cancer risk increases with age, we also review recent studies on the aged mammary gland in relation to lineage identity and provide perspectives for future research in understanding the niche of mammary epithelial cells.</p>