Abstract Drawing upon research in social psychology and international relations, this chapter elucidates how nationalistic sentiments shape mass foreign policy preferences. Specifically, it unpacks the common micro-level assumptions on which studies of nationalism and war have relied by examining the following questions: What are nationalistic sentiments? What psychological and cognitive traits do nationalistic sentiments have? How do they affect people’s foreign policy preferences? In what contexts do these sentiments cause adverse foreign policy preferences? Can those adverse preferences be easily reversed? The chapter closes by discussing why popular nationalism matters to political elites in their foreign policy decision-making and how they observe it.