Ductal carcinoma in situ (DCIS) is noninvasive breast cancer that encompasses a wide spectrum of diseases ranging from low-grade lesions that are not life threatening to high-grade lesions that may harbor foci of invasive breast cancer. DCIS is characterized histologically by the proliferation of malignant epithelial cells that are bounded by the basement membrane of the breast ducts. DCIS has been classified according to architectural pattern (solid, cribriform, papillary, and micropapillary), tumor grade (high, intermediate, and low grade), and the presence or absence of comedo histology. Prior to the advent of widespread screening mammography, DCIS was usually diagnosed by surgical removal of a suspicious breast mass. DCIS was rarely diagnosed before 1980, but currently about 25 percent of breast cancers diagnosed in the United States are DCIS. The incidence of DCIS has risen from 1.87 per 100,000 women from 1973-1975 to 32.5 per 100,000 in 2004. The incidence of DCIS increased in all age categories with the greatest rise among those older than 50 years of age. Age adjusted DCIS incidence rates increased 7.2-fold from 1980 to 2004. The annual incidence among those older than 50 years of age demonstrated an exponential increase from five per 100,000 in 1980 to 59-77 per 100,000 in 2004. While other countries have also observed increases in DCIS in recent years, no country has experienced as steep an increase in DCIS as the United States. The increase in DCIS has not, however, been uniform across histologic types. Comedo histology is associated with a particularly high risk of recurrence and has been stable over recent years. In contrast, low-grade DCIS, generally considered to be less likely to recur or develop into invasive breast cancer, has accounted for the majority of the recent increase. Many studies point to increased use of mammography as the likely explanation for the increased incidence, but the increased incidence cannot be entirely explained by an increase in screening. Cumulative incidence per 1,000 mammograms increased from 0.9 in January 1997 to 1.7 in December 2003. We assessed the impact of screening by comparing patterns of incidence using two different definitions: DCIS incidence per 100,000 female population and per 1,000 screened women. Incidence of DCIS in the United States increased over time according to both definitions. Older women had higher incidence according to both definitions. Proportional changes, when compared across the studies, tend to be larger for incidence per 100,000. The data revealed greater increases over time in incidence per 100,000 population than per 1,000 screened. Important scientific questions that deserve further investigation include gaining a better understanding of the relationship between mammography use and DCIS incidence, whether it is possible to modify current imaging technologies or screening guidelines to better identify lesions that are unlikely to become clinically problematic as well as tumors that are likely to contain some invasive component. The following proposed recommendations are organized by the original questions: Question 1. What are the incidence and prevalence of DCIS and its specific pathologic subtypes, and how are incidence and prevalence influenced by mode of detection, population characteristics, and other risk factors? Question 2. How does the use of MRI or SLNB impact important outcomes in patients diagnosed with DCIS? Question 3. How do local control and systemic outcomes vary in DCIS based on tumor and patient characteristics? Question 4. In patients with DCIS, what is the impact of surgery, radiation, and systemic treatment on outcomes?