Ductal carcinoma in situ of the breast (DCIS) is an early, noninvasive stage of breast malignancy that arises from ductal epithelium and has an especially favorable prognosis. Its biologic characteristics are consistent with a direct precursor to invasive carcinoma, which develops in the majority of cases if left untreated, generally within 10 years of diagnosis. Mammography has resulted in a substantial increase in its diagnosis, as well as a change in its presentation from large, palpable masses to nonpalpable lesions manifested primarily as microcalcifications. The same treatment options are available for DCIS as for invasive breast carcinoma, and there is also a limited role for wide local excision alone in incidental lesions. Most cases of DCIS currently are treated effectively by lumpectomy and radiation therapy, although the fact that 50% of all local breast recurrences are invasive lesions may affect survival adversely. Mastectomy is associated with the best survival rates and should be performed on any patient with factors known to pose a high risk of locoregional recurrence. There are still many outstanding issues to be resolved by further study before the intriguing potential of this disease can be realized fully.