Demographic study of the Jews in the former Soviet Union (FSU), based on a wealth of statistical data, has a long and well-established tradition. This essay first presents an overview of tsarist and Soviet demographic data regarding Jews. Because most of the Soviet data were kept hidden until the end of the Soviet period, the focus here is on findings of the last quarter century. Particular attention will then be given to the role of the Soviet internal passport (which, because it listed ethnicity, was the basis for Jewish statistics) and to the consequences of the elimination of compulsory ethnic identification in the post-Soviet Slavic countries.