How could Germans, inhabitants of the most scientifically advanced nation in the world in the early 20th century, have espoused the inherently unscientific racist doctrines put forward by the Nazi leadership? Eric Ehrenreich traces the widespread acceptance of Nazi policies requiring German individuals to prove their Aryan ancestry to the popularity of ideas about eugenics and racial science that were advanced in the late Imperial and Weimar periods by practitioners of genealogy and eugenics. After the enactment of Nazi racial laws in the 1930s, the Reich Genealogical Authority, employing professional genealogists, became the providers and arbiters of the ancestral proof. This is the first detailed study of the operation of the ancestral proof in the Third Reich and the link between Nazi racism and earlier German genealogical practices. The widespread acceptance of this racist ideology by ordinary Germans helped create the conditions for the Final Solution.