‘The theory of communism’ examines those aspects of communist theory that provide a better understanding of how Communists in power perceived the world, why they acted as they did, and how they attempted to justify their actions. It focuses on Marxism, Leninism, and Stalinism. Communist theory is shown to be ambiguous, often incomplete, and sometimes contradictory. This is partly because the various theorists were writing at different times about different conditions and in different personal situations. However, the fundamental differences of approach amongst communist theorists can also be explained in terms of the voluntarism versus determinism debate.