DCIS is a heterogeneous disease whose natural history is poorly defined. Screening mammography has increased the detection rate of DCIS, but we remain unable to identify cases of DCIS that will not progress to invasive carcinoma during an individual's lifetime. Genomics holds great promise in this regard, but prospective studies with long-term follow-up will be needed before concluding that a subset of DCIS is clinically insignificant. The varying intensity of treatment options for DCIS, ranging from mastectomy to excision, RT, and tamoxifen to excision alone, reflects the uncertainty about the natural history of DCIS as well as differing physician values regarding the impact of local recurrence. The extent of DCIS within the breast is the major determinant of whether the patient is a candidate for a breast-conserving approach, and contraindications to the use of breast conservation treatment and to the use of irradiation have been defined. The clinical decision-making process in DCIS would benefit greatly from improvements in our ability to convey information about the long-term risks and benefits of therapy, as well as the tradeoffs in health-related quality of life, to patients, and to incorporate their preferences into the decision-making process.