If there were a list of subjects about which it seems that there is definitely no more to be said—or that even if there were, nobody would want to hear it anyway—then surely the topics of ideology, in general, and ideology in Marxist theory, in particular, would have to rank very near the top. Diffusing outwards from Western Marxism's postwar preoccupation with the mechanisms of capitalist hegemony, the concept has now become virtually ubiquitous in the humanities and social sciences. Yet we feel reluctantly impelled to add one more paper to this already overflowing pile, hoping to catch readers' attention even as reflexive ennui glazes over their eyes. For a (though not, we trust, the only) virtue of the thesis we are going to propose is its originality in relation to the contemporary spectrum of opinion. If we are right, then, in one of Marx and Engels' main uses of the expression “ideology” they were not referring to ideas at all, but to the social superstructure itself. Even on purely scholarly grounds, it would be of some interest that so many commentators could have missed this meaning of “ideology” in texts as thoroughly scrutinized as these have been. But more importantly, if our analysis is correct, then certain very famous passages in the Marxist corpus (such as the 1859 “Preface” to A Contribution to the Critique of Political Economy and parts of the Critique of the Gotha Programme) have been misinterpreted for decades.