Biology of breast carcinoma appears an integral composite of various proliferative and infiltrative events that characteristically are acquired through a malignant transformation event. Grading and staging of a given neoplasm are biologic attributes of the lesion that would incorporate not only proliferation and stromal infiltration but also a full range of inter- reactive changes also contributing to the formation of the lesion. Attempts at recapitulation of the native breast tissues appear to fall short of actual ductal differentiation in many breast carcinomas and this appears essentially reflected in an infiltration of the proliferating stroma that reacts desmoplastically to the neoplastic cells. It is in such terms that breast lesions ranging from adenosis to hyperplasias of variable atypia would relate to a stroma that proliferates concurrently with the epithelial component, as prominently seen with fibroadenomas. Intraductal epithelial lesions would attest to a differentiation of ductal-type structures. The role of hormonal cyclical activity would determine parameters of concurrent proliferation of the stroma and ductal epithelium in further progressing to a potentially infiltrative lesion.