This chapter introduces the case of Marcel Déat, a leader of the French Socialist Party who founded one of the main collaborationist parties during the German occupation of France. I define political conversion and argue that accounts of it tend to be marked by a continuity bias. This is true of extant accounts of Déat’s conversion, which emphasize the significance of neo-socialism in determining his trajectory. I argue that this explanatory emphasis on ideological continuity is theoretically and empirically unsound. As an alternative approach, I introduce what I call the practical logic of political conversion, drawing on anti-essentialist and relational theories of political ideologies that treat these as articulated products of classification struggles within political fields. I argue further that the fundamentally discontinuous and relational character of political conversion means that it should be analyzed in terms of what Bourdieu calls “trajectory.” I end with an overview of the book.