The aim of this thesis is to explicate the concepts of ideology and science; in particular, social science, in the work of Marx and in later Marxism. These concepts are standardly discussed in close association with each other in both Marxist and non-Marxist contexts. Yet, in the literature on Marxism at any rate, the kind of significance they have for each other has been widely misunderstood. Historically the most important expression of such misunderstanding is the tendency in writings influenced by, or within the ambit of, 'Western Marxism' to assume that the central question concerns the precise nature of the distinction between science and ideology as rival or alternative forms of cognition. Answers to this question have generally sought to distinguish them in terms of cognitive success and failure, with ideology as the dark shadow or distorted 'other' of science. It will be shown that in relation to Marx's thought this mode of question and answer is wholly misconceived. Science and ideology function there as categorially diverse notions which are such that the problem of how to demarcate their individual shares of a common field of reference cannot arise. Problems that do pressingly arise in connection with this body of thought include the following. How does science succeed in being ideological, when it does? More specifically, how is the ideological status of Marx's own social science to be conceived? To answer these questions one has to recognise the diversity of the ways in which ideology operates and to devise theoretical models which can capture that diversity. In what follows, three basic models, labelled for convenience 'semantic', 'syntactic', and 'dialectical', will be distinguished. It will be argued that the dialectical model is the appropriate one for understanding Marx's social science. The account of the concept of ideology given here is based on that contained in my book The Real World of Ideology. ' The account there has been abridged in order to bring out what are for present purposes its essential features. A copy of the book is enclosed with the thesis. The basic plan of the thesis is as follows. The first chapter is concerned, as an essential preliminary, with explicating the conception of ideology that operates in the work of Marx and in 'classical Marxism'. The second chapter deals with a basic misconception of this legacy which has a special relevance in the present context. This is the idea that ideology is essentially to be understood as an epistemological category. The way is then clear to pose the question of the nature and status of Marx's social science. This is done in the third chapter, where it also proves possible to dispose of the claims of the syntactic model as the basis for an answer. The semantic model is a much more serious candidate and requires extended discussion. The paradigmatic version of the model in Western Marxism is the conception of Marxist social science as a 'critical theory of society'. The fourth chapter discusses the prototype of such interpretations in the work of the 'Frankfurt School' theorists. The next two chapters (5 and 6) deal with more recent attempts by writers in the British analytical tradition to vindicate the project of Marxist social critique. The failure of the project in all these versions clears the ground for an inquiry into the claims of the dialectical model. This is pursued in chapters 7 and 8, paying close attention to the evidence on the subject yielded by Marx's writings. The result is to establish the dialectical scheme as the basic instrument for understanding his social theory. In the final chapter developments treated earlier at the level of relations between ideas are set in a historical context. This enables the dialectical thesis to be grasped in a richer, more solid setting, and enables it in turn to shed its light on the question of the overall shape of the Marxist intellectual tradition.