This chapter explains and defends the sort of materialism that was a core theoretical commitment of Marx and of the early Frankfurt School—in particular, Max Horkheimer—and which has been the subject of aggressive internecine criticisms on the part of later critical theorists, like Jürgen Habermas and Axel Honneth. The author offers grounds for thinking Marx's and Horkheimer's materialism both attractive and plausible. Section 10.1 establishes some necessary distinctions between substantive and explanatory materialism, Sect. 10.2 offers an account of Marx's materialism, intended to hew closely to Horkheimer's reading of Marx, Sect. 10.3 discusses Horkheimer's understanding of materialism as a practical–theoretical commitment, and Sect. 10.4 concludes the chapter by offering some defense of materialism, chiefly centered around the normative importance of social labor, which both Marx and Horkheimer affirm.