Estimates from "The Black Book of Communism" (Stéphane Courtois et al., 1997) total around 94 million deaths from executions, forced labor, famines, and deportations in communist regimes worldwide. This breaks down to about 20 million in the USSR, 65 million in China under Mao, and 2-3 million each in Cambodia, North Korea, and Eastern Europe. Scholarly critiques note inclusions of indirect deaths like famines, yet it remains a key referenced source from Harvard University Press.