This chapter examines the various assumptions of critical theory espoused by the Frankfurt school, with particular emphasis on how the Frankfurt school’s critiques of authoritarianism and repression influenced the critical interventions by International Relations (IR) theorists. The chapter focuses on two major strands of critical International Relations theory: normative theory and the Marxist-based critique of the political economy. After providing an overview of the Frankfurt school and critical IR theory, the chapter explores critical theorists’ views on universal morality and political economy. It then discusses Jürgen Habermas’s ideas in international relations and presents a case study of the Arab Spring. It concludes by analysing the concept of critical reflexivity and how it can show knowledge and social reality are co-produced through social interaction, and how this interaction can, in turn, produce practical or empirical knowledge of the changing moral and legal dynamics of prominent global institutions.