The International Agency for Research on Cancer (IARC) has updated its 2002 guidelines on screening for breast cancer, drawing on data from studies completed in the past 15 years. In November 2014, experts from 16 countries met at the International Agency for Research on Cancer (IARC) to assess the cancer-preventive and adverse effects of different methods of screening for breast cancer. (The members of the working group for volume 15 of the IARC Handbook are listed at the end of the article; affiliations are provided in the Supplementary Appendix, available with the full text of this article at NEJM.org.) This update of the 2002 IARC handbook on breast-cancer screening 1 is timely for several reasons. Recent improvements in treatment outcomes for late-stage breast cancer and concerns regarding overdiagnosis call for reconsideration. . . .