Major efforts have been recently devoted to a better definition of intraductal proliferative lesions with atypia. The new WHO classification of tumors of the breast highlights the morphological features of flat epithelial atypia (DIN 1A) and atypical duct hyperplasia (DIN 1B). Flat epithelial atypia now encompasses lesions previously designated as clinging carcinoma (monomorphous type) and atypical columnar changes. Atypical ductal hyperplasia (ADH) is characterized by the same cytological changes as low-grade DCIS, involving a very small portion of the ductal tree. Minimal lymph node involvement includes true micrometastases (from 0.2 to 2 mm in size) and isolated tumor cells (ITC). ITC have been defined as individual tumor cells or small clusters of cells, not more than 0.2 mm in size, that do not typically show evidence of metastatic activity or penetration of vascular or lymphatic sinus walls. The biological and clinical implications of ITC remain to be elucidated.