How was the Gestapo able to detect the smallest signs of non-compliance with Nazi doctrines—especially "crimes" pertaining to the private spheres of social, family, and sexual life? How could the police enforce policies such as those designed to isolate Jews, or the foreign workers brought to Germany after 1939, with such apparent ease? Addressing these questions, Gellately argues that the key factor in the successful enforcement of Nazi racial policy was the willingness of German citizens to provide authorities with information about suspected "criminality." He demonstrates that without some degree of popular participation in the operation of institutions such as the Gestapo, the regime would have been seriously hampered in the "realization of the unthinkable," not only inside Germany but also in many of the occupied countries. Offering an intriguing examination of the everyday operations of the Gestapo and the product of extensive archival research, this incisive study surveys the experiences of areas across Germany, drawing out national, local, and regional implications.