At a time when many repentant leftists are proclaiming Marxism incapable of explaining the new phenomena of the last quarter of the twentieth century, Ernest Mandel reminds us that Marxism drew from its very inception on the advances of all the social sciences and emancipation movements of its time. In a survey of the multiple sources of Marx and Engels' theory, Mandel identifies the specific contribution of the two friends in the various disciplines to which they applied themselves: philosophy, political economy, social history, revolutionary organisation, self-organisation of the working class, emancipation movements, and internationalism. Concluding that Marxism 'constantly learns from perpetually changing reality' and that it is the conscious expression of the real movement of workers toward self-emancipation, Mandel proposes a formula which provides for a dialectical interaction between innovation and the verification of established tenets.