The period of Stalin's purges began in December 1934 when Sergei Kirov, a popular Leningrad party chief who advocated a moderate policy toward the peasants, was assassinated. Although details remain murky, many Western historians believe that Stalin instigated the murder to rid himself of a potential opponent. In any event, in the resultant mass purge of the local Leningrad party, thousands were deported to camps in Siberia. Zinov'ev and Kamenev, Stalin's former political partners, received prison sentences for their alleged role in Kirov's murder. At the same time, the NKVD, the secret police, stepped up surveillance through its agents and informers and claimed to uncover anti-Soviet conspiracies among prominent long-term party members. At three publicized show trials held in Moscow between 1936 and 1938, dozens of these Old Bolsheviks, including Zinov'ev, Kamenev, and Bukharin, confessed to improbable crimes against the Soviet state and were executed. (The last of Stalin's old enemies, Trotsky, who had supposedly masterminded the conspiracies against Stalin from abroad, was murdered in Mexico in 1940, presumably by the NKVD.) Coincident with the show trials against the original leadership of the party, unpublicized purges swept through the ranks of younger leaders in party, government, industrial management, and cultural affairs. Party purges in the non-Russian republics were particularly severe. The Ezhovshchina ("era of Ezhov," named for NKVD chief Nikolai Ezhov) ravaged the military as well, leading to the execution or incarceration of about half the entire military officer corps. The secret police also terrorized the general populace, with untold numbers of common people punished for spurious crimes. By the time the purges subsided in 1938, millions of Soviet leaders, officials, and other citizens had been executed, imprisoned, or exiled.

The reasons for this period of widespread purges remain unclear. Western historians variously hypothesize that Stalin created the terror out of a desire to goad the population to carry out his intensive modernization program, or to atomize society to preclude dissent, or simply out of brutal paranoia. Whatever the causes, the purges must be viewed as a counterproductive episode that weakened the Soviet state.

In 1936, just as the purges were intensifying the Great Terror, Stalin approved a new Soviet constitution to replace that of 1924. Hailed as "the most democratic constitution in the world," the 1936 document stipulated free and secret elections based on universal suffrage and guaranteed the citizenry a range of civil and economic rights. But in practice the freedoms implied by these rights were denied by provisions elsewhere in the constitution that indicated that the basic structure of Soviet society could not be changed and that the party retained all political power.

The power of the party, in turn, now was concentrated in the persons of Stalin and his handpicked Politburo. Symbolic of the lack of influence of the party rank and file, party congresses met less and less frequently. State power, far from "withering away" after the revolution as Karl Marx had predicted, instead grew in strength. Stalin's personal dictatorship found reflection in the adulation that surrounded him; the reverence accorded Stalin in Soviet society gradually eclipsed that given to Lenin.