The mouse has emerged as a primary animal model for human breast cancer because the mammary glands of the two species are very similar in structure and function. In this regard the TDLU and LA have similar morphology. The mouse, infected by MMTV, develops "spontaneous" tumors with specific but limited tumor phenotypes. The advent of genetic manipulation has created transgenic mice that develop hyperplasias and tumors morphologically and cytochemically comparable to lesions in humans. Even experienced pathologists have difficulty distinguishing between lesions from the two species, and the morphological similarities support the utility of the mouse model in understanding human breast cancer. In this essay we review our experience with the histopathology of human and mouse mammary disease by comparing the normal gland with hyperplastic, dysplastic and neoplastic lesions of traditional and transgenic origin.