Counting conservatively, data show about 100 million mass atrocity-related deaths since 1900. A distinct empirical phenomenon, mass atrocities are events of enormous scale, severity, and brutality, occur in wartime and in peacetime, are geographically widespread, occur with surprising frequency, under various systems of governance, and can be long-lasting in their effects on economic and human development, wellbeing, and wealth, more so when nonfatal physical injuries and mental trauma also are considered. As such, mass atrocities are a major economic concern. Given the multidisciplinary nature of the subject matter, the pertinent conceptual, theoretical, and empirical literatures are voluminous and widely dispersed, and have not been synthesized before from an economics point of view. We address two gaps: a “mass atrocities gap” in the economics literature and an “economics gap” in mass atrocities scholarship. Our goals are, first, to survey and synthesize for economists a broad sweep of literatures on which to base further work in this field and, second, for both economists and noneconomists to learn how economic inquiry contributes to understanding the causes and conduct of mass atrocities and, possibly, to their mitigation and prevention. In drawing on standard, behavioral, identity, social network, and complex systems economics, we find that the big puzzles of the “how” and “why” of mass atrocities, and mass participation therein, are being well addressed. While new research on such topics will be valuable, work should also progress to develop improved prevention approaches. (JEL D72, D74, K38, N40, Z13)