This chapter lays the intellectual groundwork for understanding relations between and within groups, exploring key psychological theories in intergroup relations and group processes. We discuss psychological theories and concepts such as evolution, relative deprivation, Realistic Conflict Theory (RCT) and Social Identity Theory (SIT), and contextualise the importance of intergroup relations by addressing their darker elements (e.g. prejudice, discrimination and conflict) and their potential for fostering peace and cooperation. The chapter then outlines key concepts in the study of group processes, discussing how groups act collectively and how norms are formed. It explores the role of social identity in individuals’ alignment with group norms and how leadership works from both individual and group-based perspectives. While individuals make decisions, groups act too, and group dynamics constrain or empower individuals’ agency. The chapter concludes by preparing readers to understand how new information, norms and leader-follower relations collectively drive system change, setting the stage for the following chapters.