This is first detailed account on the origins of critical theory and how it has developed and influenced international relations. Making complex ideas accessible to all, it discusses Marx and post-Marxist thinkers; the Frankfurt School; Adorno, Benjamin and Marcuse; Habermas, Gramsci and Foucault alongside critical IR scholars Robert W. Cox, Rob Walker, James Der Derian, Christine Sylvester, Richard Ashley and Andrew Linklater.