Most types of invasive breast cancer are thought to evolve over long periods from specific preexisting benign lesions. Of the many types of benign entities found in the human breast, only a few have clinically significant premalignant potential. Currently, the best-characterized premalignant lesions are atypical ductal hyperplasia, atypical lobular hyperplasia, and lobular carcinoma in situ. Ductal carcinoma in situ is considered to be a preinvasive malignant lesion. Two additional lesions, unfolded lobules and usual ductal hyperplasia, are sometimes considered to be very early premalignant epithelial abnormalities. Premalignant lesions are currently defined by their histologic features, and not all necessarily progress to invasive cancer. This suggests that although lesions within specific categories look alike, they must possess underlying genetic differences that cause some to remain stable and others to advance. The development of modern molecular genetic techniques has allowed breast cancer researchers to clarify the multistep model of breast carcinogenesis. Recent studies indicate that cancer evolves by highly diverse genetic mechanisms, and research into these altered pathways may identify specific early defects that might be targeted to prevent progression of premalignant lesions to invasive cancer. Current clinical management is heterogeneous and depends on histologic examination and individual patient factors. Options for breast cancer risk reduction and prevention are available.