This work traces the changes in classical Marxism (the Marxism of Karl Marx and Friedrich Engels) that took place after the death of its founders. It outlines the variants that appeared around the turn of the twentieth century--one of which was to be of influence among the followers of Adolf Hitler, another of which was to shape the ideology of Benito Mussolini, and still another of which provided the doctrinal rationale for V. I. Lenin's Bolshevism and Joseph Stalin's communism. This account differs from many others by rejecting a traditional left/right distinction--a distinction that makes it difficult to understand how totalitarian political institutions could arise out of presumably diametrically opposed political ideologies. Marxism, Fascism, and Totalitarianism thus helps to explain the common features of "left-wing" and "right-wing" regimes in the twentieth century.