The definition of atypical ductal hyperplasia (ADH) encompasses qualitative and quantitative criteria. Qualitative criteria include cytological and architectural features similar to those of low grade ductal carcinoma in situ (DCIS), the quantitative criteria are characterized by metric features (2 mm or 2 ductules) or by the confines of lobules. In this article we discuss the morphology of ADH, the status of ADH in the low grade pathway of breast carcinoma development and its clinical significance. Furthermore, we comment some special forms of atypical epithelial proliferations of the ductal type.