Ductal carcinoma in situ of the breast (also called intraductal carcinoma), a clonal proliferation of malignant-appearing cells within the mammary duct lumens without evidence of invasion beyond the epithelial basement membrane, is the precursor lesion of invasive breast cancer. In the past 20 years, concomitant with the wide use of screening mammography, its detected incidence has risen dramatically. Data from large cohort studies and randomized trials have emerged to guide treatment. This review summarizes progress in the understanding, pathogenesis, and treatment of ductal carcinoma in situ.