The microanatomy and development of the mammary gland are unique and a reflection of its function to synthesize and deliver milk to the newborn offspring. The uniqueness of the mammary gland resides in several factors. First, the mammary parenchyma undergoes the vast majority of its growth postpubertally, thus enabling experiments on development to be performed in the juvenile or adult and presenting opportunities for experimental manipulation of the gland not available with other organs. On the basis of this characteristic, the fat pad transplantation method was developed, which resulted in the elaboration of important concepts in senescence, immortalization, and preneoplasia. Second, the accessibility of the gland and the ductal organization allows delivery and localization of specific molecules to mammary parenchyma cells, the cells which are the site of origin of neoplastic development. Third, the organ is the target of viral, chemical, and physical carcinogens, allowing development of unique and complex models for neoplastic development. Finally, the complexity of hormone and growth factor regulation of mammary gland function allows a sophisticated approach to the study of hormone action. The purpose of this review is to illustrate some unique properties of the gland which provide the basis for specialized approaches to developmental, neoplastic, and functional problems.