In situ carcinoma of the breast is a proliferation of potentially malignant cells within the lumen of the ductal-lobular system and is classified as ductal or lobular in type. It has become an increasingly frequent clinical management problem, primarily because of its enhanced detection by screening mammography. In this paper, we discuss the problems in histologic diagnosis, the natural history, presentation, and options for management of these two forms of in situ carcinoma.