This dissertation is a comparative and transnational history of Fascist and Nazi spies from the creation of the Fascist secret police in 1927 to the end of the Second World War in 1945. Who spied for the totalitarian dictatorships and why? How does a history of these largely unknown figures reorient scholarship on the period? I conceptualize totalitarianism from the bottom up: through in-depth microhistorical analysis of the motives and milieux of more than fifty spies, “Lives Under Cover” depicts the inner workings of the Fascist and Nazi bureaucracies of terror. While previous historiographies have concentrated on the police chiefs and political leaders rather than on their agents in the field, I renovate our understanding of totalitarianism, by tracking the infiltration of networks of political opponents by Fascist and Nazi agents and comparing their activity in Fascist Italy, Nazi Germany, Nazi Austria, and Vichy France.Drawing on a variety of archival sources, such as Fascist police records and court trials of convicted Nazi spies, I explain how the secret police spies at times enforced and at times disrupted the totalitarian system of repression. Scholars provide us with insight into the institutional structures of the Fascist and the Nazi secret police. Nevertheless, we are left with the misleading impression that these police services were perfectly functioning machines of terror controlled by a small number of powerful people. I build on this scholarship by moving the focus to the individuals who were tasked with enforcing these systems. My research contributes to the history of authoritarian repression in that I introduce a new point of view by exploring the liminal state between prosecutors and victims that the spies inhabited. Fascist and Nazi surveillance systems needed to incorporate some of the very groups they excluded and dominated, such as gay and Jewish people. I argue that these surveillance systems depended on individuals from these groups to gain access to marginal spaces that they could not otherwise infiltrate. At the same time, however, relying on informers made these systems more fragile, because the spies pursued their own agendas and were, at times, both targets and actors of the totalitarian repression.