Studies of mammary gland biology are essential in the fight against breast cancer. Moreover, the mammary gland represents an excellent model system for investigation of physiological and pathological processes that occur throughout the body. The branching morphogenesis that occurs during puberty and the alveogenesis processes during pregnancy, for example, recapitulate steps that occur in many tissues during embryogenesis and are also relevant to tumor invasion and metastasis. Similarly, analyses of breast cancer progression and the cell types that contribute to it yield findings applicable not only to breast cancer but widely to other cancers as well. Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology reviews our understanding of mammary gland development, physiology, and tumor formation, emphasizing the value of the organ as a model system. The contributors examine the roles of hormones, growth factors, oncogenes, and the extracellular matrix in normal development and cancer. In addition, they discuss the nature of the mammary gland stem cell population and review the gene expression programs activated and chromatin modifications that occur as the different cell lineages present differentiate. The book also contains chapters covering the many experimental approaches that have been used in this field, including transplantation, genetic engineering, RNA interference and genomics, three dimensional culture models, and live cell imaging. It will thus be of interest to all researchers working in cell, developmental, and cancer biology, as well as those focused specifically on mammary gland biology and breast cancer.