This paper undertakes an historical re‐evaluation of Louis Althusser's philosophical legacy for modern Marxism. While Althusser self‐consciously sought to defend the scientific character of Marxism, many of his closest followers eventually exited the Marxian paradigm for a post‐structural post‐Marxism. We argue that this development was predominately rooted in a series of philosophical errors that proved fatal in a period of retreat for European socialism. There has always been, however, a second post‐Althusserian legacy associated with the critical realist conception of Marxism initiated by Roy Bhaskar. Bhaskar found part of his inspiration in Althusser's successful posing of the question of Marx's science and this paper sets out to excavate the proper links between Althusser and Bhaskar in order to deepen the relationship between critical realism and scientific Marxism.