A revised version of the book, called The Great Terror: A Reassessment, was printed in 1990 after Conquest was able to amend the text, having consulted recently opened Soviet archives.
One of the first books by a Western writer to discuss the Great Purge in the Soviet Union, it was based mainly on information which had been made public, either officially or by individuals, during the Khrushchev Thaw in the period 1956–1964. It also drew on accounts by Russian and Ukrainian émigrés and exiles dating back to the 1930s. Lastly it was based on an analysis of official Soviet documents such as the census.
The first critical inquiry into the Great Purge outside USSR had been made as early as 1937, by the Dewey Commission, which published its findings in the form of a 422-page book entitled Not Guilty (this title referred to the people who had been charged with various crimes by Stalin's government and therefore purged; the Dewey Commission found them not guilty). The most important aim of Robert Conquest's The Great Terror was to widen the understanding of the purges beyond the previous narrow focus on the "Moscow Trials" of disgraced Communist Party leaders such as Nikolai Bukharin and Grigory Zinoviev. The question of why these leaders had pleaded guilty and confessed to various crimes at the trials had become a topic of discussion for a number of western writers, and had underlain books such as George Orwell's Nineteen Eighty-Four and Arthur Koestler's Darkness at Noon. According to the book, the trials and executions of these former Communist leaders were a minor detail of the purges, which, together with man-made famines, had led to 20 million deaths according to his estimates. In the appendix of the original 1968 edition of The Great Terror, Conquest estimated 700,000 "legal" executions took place during 1937 and 1938, which was roughly confirmed by the 681,692 executions found in the Soviet archives for these two years. In the preface to the 40th anniversary edition of The Great Terror, Conquest wrote that he had been "correct on the vital matter—the numbers put to death: about one million", but lowered other figures saying the total number of deaths brought about by the various Soviet terror campaigns "can hardly be lower than some fifteen million."
In the book, Conquest disputed the assertion made by Nikita Khrushchev, and supported by many Western leftists, that Stalin and his purges were an aberration from the ideals of the Revolution and were contrary to the principles of Leninism. Conquest argued that Stalinism was a natural consequence of the system established by Lenin, although he conceded that the personal character traits of Stalin had brought about the particular horrors of the late 1930s. Neal Ascherson noted: "Everyone by then could agree that Stalin was a very wicked man and a very evil one, but we still wanted to believe in Lenin; and Conquest said that Lenin was just as bad and that Stalin was simply carrying out Lenin's programme."
In the book Conquest sharply criticized Western intellectuals for their blindness towards the realities of the Soviet Union, both in the 1930s and, in some cases, even in the 1960s. He described figures, such as Beatrice and Sidney Webb, George Bernard Shaw, Jean-Paul Sartre, Walter Duranty, Sir Bernard Pares, Harold Laski, D. N. Pritt, Theodore Dreiser and Romain Rolland as dupes of Stalin and apologists for his regime for denying, excusing, or justifying various aspects of the purges.
A widespread story recounts that when Conquest was asked to provide a new title for an anniversary edition, after his initial findings were verified by the opened Soviet archives, he replied, "How about I Told You So, You Fucking Fools?" According to Conquest, this never happened, and was a joking invention of writer Kingsley Amis.
After the collapse of the Soviet Union and the opening up of the Soviet archives, many of Conquest's claims were validated as having been accurate. However, Conquest's estimates of the death toll and other aspects of his research were challenged by many historians, such as Arch Getty, Trotskyist historian Vadim Rogovin, and German historian Gábor T. Rittersporn.