The mouse has emerged as a primary animal modelfor human breast cancer because the mammary glands ofthe two species are very similar in structure andfunction. In this regard the TDLU⁴ and LAhave similar morphology. The mouse, infected by MMTV,develops "spontaneous" tumors with specificbut limited tumor phenotypes. The advent of geneticmanipulation has created transgenic mice that develophyperplasias and tumors morphologically and cytochemicallycomparable to lesions in humans. Even experiencedpathologists have difficulty distinguishing betweenlesions from the two species, and the morphologicalsimilarities support the utility of the mouse model inunderstanding human breast cancer. In this essay wereview our experience with the histopathology of humanand mouse mammary disease by comparing the normal gland with hyperplastic, dysplastic and neoplasticlesions of traditional and transgenic origin.