Breast cancer overdiagnosis, the detection of tumors unlikely to become clinically apparent in the absence of screening, is an important, but elusive, concept. It is important because the diagnosis and treatment of invasive breast cancer and high-risk noninvasive lesions, particularly ductal carcinoma in situ (DCIS), involve potentially harmful procedures and therapies and have considerable psychosocial impacts on women. Because most diagnoses of high-risk lesions, DCIS, and early invasive breast cancer follow screening mammography, overdiagnosis is clinically relevant to millions of women undergoing screening in the United States each year.