Incidence of ductal carcinoma in situ (DCIS) has increased significantly during the last decade, comprising almost 20% of all breast cancers diagnosed today. DCIS is composed of malignant breast duct epithelial cells that have clonally proliferated and accumulated within the mammary duct lumen. It comprises a group of heterogeneous tumors with varying biologic behavior rendering its classification and management challenging. By definition, DCIS does not invade through the basement membrane; it is a preinvasive malignancy and systemic disease is nonexistent. The basis of treatment is to prevent progression into an invasive cancer such as ductal carcinoma. Most current DCIS classification schemes do not predict its potential to progress to invasive disease. In this review the natural history of DCIS, as it relates to its biologic potential to progress to invasive disease, is summarized, and a broad classification system that may provide a guideline for patient management is proposed.