Karl Marx’s philosophical, sociological, economic and political writings have had a deep impact on the practice of politics and international politics during the past two centuries. They have also had far-reaching inﬂuence on critical social theorising: Marx’s thought has served as both the bedrock and the primary focus of theoretical challenge for most twentieth century ‘critical theorists’. Yet, while the inﬂuential nature of Marx’s thought is in no doubt, the precise nature of his legacy has remained contested. Various contrasting interpretations – from sympathetic ‘humanist’ readings to various ‘deterministic’ readings – have been advanced, each interpretation carrying with it important theoretical, rhetorical and political ramiﬁcations. Because of the diﬃcult interpretational problems associated with Marx’s writings, it would be impossible to provide a deﬁnitive interpretation of Marx’s ideas here. The focus here, rather modestly, is on, ﬁrst, giving a brief account of the context of Marx’s writings before proceeding to outline some of the key concepts associated with his work. I conclude by reﬂecting brieﬂy on the legacy of Marx for twentieth century critical social theory.