Lobular and ductal intraepithelial neoplasias reflect proliferations of immunophenotypically variable, biologically and morphologically diverse cells with a potential, not always realized, for progression to carcinoma by breaking through the barriers of the myoepithelial cell layer and basement membrane, ultimately invading the stroma. Starting with the lobular and then the ductal proliferations, this review will address the evolution of our understanding of these lesions; the problems associated with the conventional terminology of ductal hyperplasia, atypical hyperplasia, and carcinoma in situ; and reasons for and advantages of the intraepithelial neoplasia terminology.