The emergence of nation states is a relatively recent phenomenon in human history. Yet its impact on everyday life is ubiquitous. The purpose of the present article is to synthesise research from several social science disciplines to identify similarities and differences between legal and structural definitions of nation states studied by political scientists and historians and psychological conceptions of nation states studied by social psychologists. Using a social psychological lens, we investigate how nation states as political institutions influence psychological conceptions of national identity and how these construals have unique effects on perceivers’ attitudes, behaviour, and inclusion of diverse ethnic groups within the nation. Four research questions guide this article. First, how do modern nation states define citizenship legally and to what extent do these definitions fit psychological conceptions of nationality that individuals report explicitly or implicitly? Second, to what extent do these implicit and explicit conceptions of national identity influence majority group members’ actions and decisions in both positive and negative directions? Third, what types of perceiver characteristics (e.g., national identification, political ideology, status, social dominance) influence attitudes, behaviour, and inclusion of ethnic minorities and immigrants? And finally, in what ways do conceptions of nationality impact the self-concept and well-being of ethnic minorities and immigrants within a nation? We summarise extant research that addresses each question and conclude by identifying unanswered questions and avenues for future work.