The development of the Tsar's autocratic powers reached a peak during the reign of Ivan IV, and he gained the sobriquet "the Terrible". Ivan strengthened the position of the Tsar to an exceptional degree, demonstrating the risks of unrestrained power in the hands of a mentally unstable individual. Although apparently intelligent and energetic, Ivan suffered from breakdowns of paranoia and depression, and his rule was disrupted by acts of intense violence.

Ivan IV became Grand Prince of Moscow in 1533 at the age of three. The Shuysky and Belsky factions of the boyars competed for control of the regency until Ivan assumed the throne in 1547. Reflecting Moscow's new imperial claims, Ivan's coronation as Tsar was a ritual modeled after those of the Byzantine emperors. With the continuing assistance of a group of boyars, Ivan began his reign with a series of useful reforms. In the 1550s, he declared a new law code, revamped the military, and reorganized local government. These reforms undoubtedly were intended to strengthen the state in the face of continuous warfare. The key documents prepared by the so-called Select Council of advisors and promulgated during this period are as follows:

Templat:Muscovite Constitution