Karl Mannheim's Ideology and Utopia appeared in English in 1936 and has subsequently dominated the debate about the sociological foundations of knowledge, belief and ideology. Although his work had caused a major controversy in Germany in the 1930s, his book was received within an English and North American context as a social science contribution to the analysis of the social roots of belief systems. His work is of importance because it was seen to be a significant challenge to Marxist approaches to ideology in capitalist societies, and the principal critics of his work were drawn from within the Marxist tradition. Mannheim's sociology of knowledge was shaped by the work of Max Weber and Georg Lukcs, but this article argues that the more important influence on Mannheim's sociology of knowledge derive from Martin Heidegger's criticism of metaphysics in Being and Time which was published in 1927. In particular, Mannheim's concept of the existential boundedness of knowledge derives from Heidegger's discussion of the collective and communal nature of being. Although Mannheim's work has been influential in the historical development of the sociology of knowledge, the problem of relativism has in recent decades been taken further than Mannheim's treatment of the problem, because feminism, anthropology and postmodernism have separately emphasized the contextual nature of knowledge. The problem of relativism which Mannheim sought to evade or resolve has become an embedded feature of contemporary political theory.