This chapter gives an overview of the Marxian conception of the contradistinction between ideology and science. It provides Malthus and Spencer as examples of the use of pseudoscience in the ideological portrayal of a socio-historical condition, capitalism, as something natural and unalterable. It then follows the development of the concept of ideology in twentieth-century Western Marxism, contrasting the Frankfurt School’s scepticism towards the possibility of disinterested science with Althusser’s insistence on the need for science to break with ideology.