Despite increasing concern about the threat of global crime, it remains difficult to measure. During the 1920s and 1930s, the League of Nations conducted the first social-scientific study of global crime in two studies of the worldwide traffic in women. The first study included 112 cities and 28 countries; researchers carried out 6,500 interviews in 14 languages, including 5,000 with figures in the international underworld. By drawing on archival materials in Geneva and New York, this article examines the role of ethnography in developing a social-science measure of global crime threats. The discussion covers the Rockefeller grand jury and formation of the Bureau of Social Hygiene; the League's research in Europe, the Americas, and the Mediterranean; controversy concerning the use of undercover researchers; the League's research in Asia; and the end of the Bureau. The League's experience demonstrates the promise of multisite ethnography in research about global crime as well as the difficulty of mapping crime on a global scale. Published by arrangement with John Wiley & Sons.