The mammary gland is a complex organ that begins development early in gestation and constantly changes in size, shape and function from the time of puberty to menopause. The earliest stages of embryogenesis appear to be independent of steroid hormones, whereas after the 15th week breast structure is largely influenced by a variety of hormones. In most females, further breast development begins at puberty under the influence of cyclical estrogen and progesterone secretion. This process may continue into the 20s and it is enhanced by pregnancy. Growth and transcription factors contribute to the reciprocal stromal-epithelial interactions in growth, development and tumorogenesis of the mammary gland. From the embryological point of view the morphology of both mammary ductal and lobular cells results from the same developmental process. Numerous data suggest the existence of self-renewing, pluripotent mammary stem cells but their molecular characteristics and differentiation pathways are unknown. The extensive research currently being done in molecular biology and pathology, cancer genomics and proteomics will hopefully contribute to further elucidation of all the genetic and environmental factors involved in the development, differentiation, and involution of the mammary gland and this may give insight into the etiopathogenesis, early detection, treatment, and potential prevention of breast cancer.