Because of the wider use of screening mammography, ductal carcinoma-in-situ, or DCIS, once rare, is now diagnosed with increasing frequency. Important questions remain unresolved regarding the natural history, classification, and management of DCIS. Many physicians have assumed that DCIS is diffuse and regularly progresses to invasive cancer; therefore, they routinely recommend mastectomy. However, as we learn more about this lesion, it is now clear that in many cases the lesion is focal in extent, "premalignant," and curable with local procedures short of mastectomy. In this review, we describe the current state of knowledge regarding the presentation, pathology, natural history, and management of DCIS.