The concept of ideology - traditionally one of Marxism's most persuasive ideas - has recently been subjected to devastating criticism. Michele Barrett shows that Marx's own writings offer a confusing array of possible approaches to 'ideology', which the classical Marxist tradition consolidated as 'mystification that serves class interests'. Barrett locates Gramsci and Althusser as key figures in the breakdown of the classical Marxist conception - Gramsci's work presaging the separation of class, politics and ideology found in Laclau and Mouffe, and Althusser's failing to deliver an adequate approach to subjectivity. Foucault - replacing Marxism's 'economics of untruth' with his own 'politics of truth' - is examined as an exemplar of post-structuralist critiques of ideology.