To see the grisly total is staggering. In its many enthroned variations, from Lenin's 1917 revolution to the recent Marxist-Leninist regimes of Africa, communism has killed upwards of 100 million people -- 65 million in China alone. Courtois and his colleagues do not simply unfold the numbers relentlessly and numbingly. Instead, they painstakingly explore the many ways the killing was done -- from summary execution to forced deportations, from mass starvation to the gulag -- and examine its many pretexts. Arguing with the passion of former believers, they charge that communism was a criminal system. They all make the case well, although some (such as Soviet expert Nicolas Werth) write with more nuance than others. Despite Courtois' brave attempt in the conclusion, however, the authors fail to answer their own central question: Why did communism, when in power, start and stay so murderous?