Intraductal epithelial proliferations of the breast are at present classified into three groups; distinction is made histologically and clinically between usual epithelial hyperplasia and atypical ductal hyperplasia (ADH) and between ADH and ductal carcinoma in situ (DCIS). Although evidence indicates that these boundaries are not ideal on a morphological, immunohistochemical, or genetic basis, this three-tier system is accepted and used at present. The current definitions, histological features, and system of classification of ADH and DCIS are described in this manuscript.