The therapy of ductal carcinoma in situ (DCIS) is controversial but is being increasingly decided by pathologic evidence. Studies of the natural history of DCIS demonstrate that DCIS is very heterogeneous in its clinical behavior. As detailed in several reviews, studies that followed patients after biopsy alone indicate a great difference between the small noncomedo examples of DCIS and the larger comedo DCIS lesions. The currently available evidence from cases that have been treated by planned surgical excision without radiation therapy would indicate that noncomedo examples of DCIS have a low incidence of recurrence and may be adequately treated by this technique. In contrast, comedo DCIS lesions have a high propensity for recurrence despite excision and radiotherapy. This presentation will review the histopathology of DCIS and highlight the idea that we are currently in a state of transition in our understanding of DCIS. Studies supporting the stratification of DCIS by histologic pattern plus cytology and size will be contrasted with the rapidly disappearing classic posture that all DCIS is biologically similar and treatment options need not be stratified by the different subtypes or varieties of DCIS.