Ductal carcinoma in situ (DCIS) is responsible for 25% of screen-detected breast cancers. Various prognostic classifications are in use, including the Van Nuys Prognostic Index and the European Organisation for Research and Treatment of Cancer grading system (well, intermediate or poorly differentiated) based on cytonuclear pattern. This has been modified in screening programs to low, intermediate and high grade. In comparison with normal epithelium, DCIS has a tenfold increase in growth and 15-fold increase in apoptosis. Patients with extensive or multifocal DCIS need mastectomy and sentinel node biopsy, together with reconstruction, if requested. Microinvasion associated with DCIS is an indication for sentinel node biopsy. Randomized trials have confirmed the value of breast irradiation after wide excision, in terms of DCIS relapse and progression to invasive disease. Patients with estrogen receptor-positive DCIS benefit from adjuvant tamoxifen after breast-conserving surgery.