Karl Marx probably had more influence on the political course of the last century than any other social thinker. There are many different kinds of Marxism, and the Twentieth Century saw two huge Marxist states in total opposition to one another. In the West, Marxism has never presented a revolutionary threat to the established order, though it has taken root as the major theoretical critique of capitalist society in intellectual circles, and new interpretations of Marx's thought appear each year.

Peter Worsley discusses all these major varieties of Marxism, distinguishing between those ideas which remain valid, those which are contestable, and those which should now be discarded. Rather than treating Marxism purely as a philosophy in the abstract, he concentrates upon the uses to which Marxism has been put and emphasises the connections between the theoretical debates and political struggles in the real world.