In situ breast carcinoma represents an early localized stage in the development of invasive breast carcinoma, which has an especially favorable prognosis with appropriate management. Its increased frequency of diagnosis in recent years is a tribute to the success of efforts at early detection of this disease. The two histologic forms of this entity are distinct in their pathologic and behavioral characteristics, as well as in their therapeutic implications. Lobular carcinoma in situ may be viewed as a marker of risk for subsequent carcinoma and is generally managed with nonoperative observation. Ductal carcinoma in situ appears to be a more ominous lesion, which is probably a direct precursor to subsequent invasive disease, and has the same therapeutic options as invasive breast carcinoma. There are still many outstanding issues and dilemmas to be resolved before the intriguing potential of in situ breast carcinoma can be fully realized. Any physician involved in the management of breast disease must be committed to a thorough understanding of all aspects of the biology of this entity, so as to be able to rationally integrate it with the specific preferences of each patient to achieve optimal results and continue to advance our knowledge and experience.