Recent studies have highlighted the potential of analyses of genomic sharing to produce insight into the demographic processes affecting human populations. We study runs of homozygosity (ROH) in 18 Jewish populations, examining these groups in relation to 123 non-Jewish populations sampled worldwide. By using a model-based clustering method to sort ROH into three length classes—short, intermediate, and long—we examine the impact of a variety of demographic processes on genomic patterns in Jewish populations. Interestingly, we find that the portion of the genome appearing in long ROH—the length class most directly related to recent consanguinity—closely accords with demographic data gathered during the 1950s on consanguineous unions in the various Jewish groups. The dissection of ROH into length classes and the comparison to consanguinity data provide insight into a number of additional phenomena, including differences between Jewish and non-Jewish populations in ROH patterns, the relative lengths of identity-by-descent tracts in different Jewish groups, and the nature of the “population isolate” status of the Ashkenazi Jews.