Interval cancer is cancer diagnosed within the time interval of one screening round (in Norway two years) after the last screening exam. Breast interval cancer has in general less favorable prognostic and predictive characteristics than screen detected breast cancer, as they commonly are larger and of higher grade. They are usually diagnosed because of clinical symptoms. Some interval cancers are true interval cancers, not visible on the previous screening mammograms. Others are clearly visible on the screening mammograms and may be classified as missed cancers, usually due to misperception or misinterpretation of the mammograms by the reading radiologist. A third group is classified as minimal sign, with only sparse findings indicating a developing cancer, with no necessarity to be recalled. Others are occult cancers, not visible either on diagnostic mammograms or previous screening mammograms. This classification of cancer is also applicable to screen detected breast cancer. In our study, interval breast cancer and screen detected breast cancer from all 16 breast diagnostic centers in Norway are reassessed, in total approximately 2400 cases. The study is organized by the Norwegian Cancer Registry and Vestre Viken hospital. In addition to the above mentioned classification, all cases are characterized according to BI-RADS (BreastImaging ReportingAndDataSystem), and analysed with respect to histopathological features. The study has the potential to improve the sensitivity of the screening program by assessment and evaluation of mammographic and histopatological features among interval and screen detected breast cancers, in order to improve the skills of the interpreting radiologists. Additionally, the study provides a major contribution to the quality assurance in the Norwegian Breast Cancer Screening Program.