Non-invasive breast cancer is comprised of two distinct entities: lobular carcinoma in-situ (LCIS) and ductal carcinoma in-situ (DCIS). The natural history of each clinical entity is described and a biologic interpretation of the available data is offered. Lobular carcinoma in-situ is considered only a risk marker rather than a precursor for the subsequent development of invasive cancer, so that once the diagnosis is established, further operative intervention is unnecessary and serial follow-up alone is recommended. The treatment of ductal carcinoma in-situ must take into account that breast-preserving therapy is now considered optimal treatment of invasive cancer of the breast, the disease we are trying to prevent. The pitfalls of recommending treatment based on retrospective data is emphasized and the need to support clinical trials designed to determine the optimal therapeutic management of intraductal carcinoma is affirmed.