The Great Terror (1937-38) in the Soviet Union occupies a central role in the history of twentieth-century mass violence. During a sixteen-month period, the Stalin regime arrested over 1.5 million people, mostly on trumped-up charges of "counterrevolutionary" and "anti-Soviet" activity, of whom about half were summarily executed and the rest were sent to the Gulag. While we now know a great deal about the experience of victims, we know almost nothing about the perpetrators. One explanation for this lacuna is that there were no public trials-no equivalent of the postwar prosecution of Nazi war criminals-of Soviet perpetrators. Yet there were secret trials of NKVD (secret police) officials, the subject of this new book by eminent Soviet historian Lynne Viola. In what has been dubbed "the purge of the purgers," almost one thousand secret police officers were prosecuted by Soviet military courts for violations of Soviet criminal procedure. They were charged with multiple counts of fabrication of evidence, falsification of interrogation protocols, use of torture to secure "confessions," and murders during pre-trial detention of "suspects." As a rule, these trials were held in strict observance of the norms of Soviet criminal procedure and resulted in conviction, with sentences ranging from administrative reprimand to execution. The documentation generated by these trials include verbatim interrogation records and written confessions signed by perpetrators in 1937-38; testimony by victims, witnesses, and experts; and stenographic transcripts of court sessions. Arguably, these heretofore unused documents constitute the holy grail of the documentation on the Great Terror and the single, most valuable source of information on Soviet perpetrators. Although the FSB (formerly KGB) archives in Moscow forbid researchers from accessing these files, the Ukrainian archives include the criminal files of secret police officials at every level of the hierarchy in the Ukrainian Soviet Socialist Republic, a multiethnic borderland where the Great Terror was at its worst. The documents illuminate one of the darkest corners of Soviet history-the inner world of Stalin's terror and the mental cosmos of Soviet perpetrators. They allow us to eavesdrop on the conversations of NKVD men, as they discuss the practices of the Great Terror amongst themselves. They permit us to listen in on the top-secret operational meetings of the NKVD at different regional levels as it set out to launch the Great Terror. They show the preparation of false confessions. They reveal the macabre violence of the execution chamber in the blood-spattered basements of the NKVD. Most importantly, they bring us directly into the interrogation room to witness the questioning and torture of victims, where the extraction of confessions was the ultimate goal. Viola presents a series of micro-histories of the terror based on the criminal files of NKVD investigators who found themselves behind bars. She details the most prolific perpetrators of Soviet mass violence and contextualizes the secret trials. Like Christopher Browning in Ordinary Men, Viola is able to build the backgrounds of these perpetrators and to interrogate their statements as the accused. These were no " but men (and at least one woman) who chose a career in the NKVD and spent years within its cloistered culture of violence, often moving them from lower-class backgrounds up the social ladder. They were children of the Revolution and participants of the Civil War or of the forced collectivization, which conditioned them to see the violent internal purge as normal. Their trials expose the state crimes of Stalin and the NKVD in a manner never expected to be public and that will fascinate and shock even those familiar with the works on Soviet oppression.