The Frankfurt School (German: Frankfurter Schule) is a school of social theory and critical philosophy associated with the Institute for Social Research, at Goethe University Frankfurt in 1929. Founded in the Weimar Republic (1918–1933), during the European interwar period (1918–1939), the Frankfurt School initially comprised intellectuals, academics, and political dissidents dissatisfied with the contemporary socio-economic systems (capitalist, fascist, communist) of the 1930s. The Frankfurt theorists proposed that social theory was inadequate for explaining the turbulent political factionalism and reactionary politics occurring in 20th century liberal capitalist societies. Critical of both capitalism and of Marxism–Leninism as philosophically inflexible systems of social organization, the School's critical theory research indicated alternative paths to realizing the social development of a society and a nation.

The Frankfurt School perspective of critical investigation (open-ended and self-critical) is based upon Freudian, Marxist and Hegelian premises of idealist philosophy. To fill the omissions of 19th-century classical Marxism, which did not address 20th-century social problems, they applied the methods of antipositivist sociology, psychoanalysis, and existentialism. The School's sociologic works derived from syntheses of the thematically pertinent works of Immanuel Kant, Georg Wilhelm Friedrich Hegel, and Karl Marx, of Sigmund Freud and Max Weber, and of Georg Simmel and György Lukács.

Like Karl Marx, the Frankfurt School concerned themselves with the conditions (political, economic, societal) that allow for social change realized by way of rational social institutions. Their emphasis on the critical component of social theory derived from their attempts to overcome the ideological limitations of positivism, materialism, and determinism by returning to the critical philosophy of Kant and his successors in German idealism—principally the philosophy of Hegel, which emphasized dialectic and contradiction as intellectual properties inherent to the human grasp of material reality.

Since the 1960s, the critical-theory work of the Institute for Social Research has been guided by Jürgen Habermas's work in communicative rationality, linguistic intersubjectivity, and "the philosophical discourse of modernity." More recently, the "third generation" critical theorists Nikolas Kompridis, Raymond Geuss, and Axel Honneth have opposed Habermas's propositions, claiming he has undermined the original social-change purposes of critical-theory-problems, such as what should reason mean; analysis of the conditions necessary to realize social emancipation; and critiques of contemporary capitalism.