Everyday life in Nazi concentration camps is a foundational theme of Holocaust Studies. In past decades, scholars have either employed a top-down approach, treating the camps as mechanisms of alienation, dehumanization, and extermination; or they have worked from the bottom-up, focusing on the prisoners and exploring questions of collaboration and resistance, strategies of individual survival, and the struggle to retain one’s humanity. Both perspectives have offered only a partial view of the complex social world of the camps, largely ignoring the group identities prisoners themselves constructed as they engaged with each other as much as they did with the authorities. To understand how prisoners socialized and made sense of an alienating environment, this chapter explores group identity formation in the Nazi concentration camps. It focuses on the forging of social networks among Greek Jewish prisoners in Auschwitz-Birkenau sustained by a reconceived notion of nationality. Through an analysis of written and audiovisual testimonies of Sephardic Jewish survivors from the city of Salonica/Thessaloniki, the chapter examines the place of “Greece” in the discourse and social practices of Salonican Jews in Auschwitz-Birkenau. “Greece” became a boundary-marker, a central category of identification and differentiation inside the Jewish prisoners’ world.