Breast carcinogenesis is a multi-step process that initially is recognized histopathologically as a series of preinvasive stages (intermediate stages) leading to invasive carcinoma, and ultimately to metastatic adenocarcinoma. Histologically the preinvasive stages of breast neoplasia consist of a sequence of epithelial changes which include hyperplasia, atypical hyperplasia, and carcinoma in-situ (1-2). All aspects of breast carcinogenesis are better characterized in ductal adenocarcinomas (the majority of breast cancers) than in lobular adenocarcinomas of the breast, which tend to have different molecular characteristics than ductal neoplasia. The subsequent discussion focuses only on ductal neoplasia of the breast. © 2006 Springer.