member of the upper stratum of medieval Russian society and state administration. In Kievan Rus during the 10th–12th century, the boyars constituted the senior group in the prince's retinue ( druzhina) and occupied the higher posts in the armed forces and in the civil administration. They also formed a boyar council, or Duma, which advised the prince in important matters of state. In the 13th and 14th centuries, in the northeastern Russian principalities, the boyars were a privileged class of rich landowners; they served the prince as his aides and councillors but retained the right to leave his service and enter that of another prince without losing their estates.

From the 15th to the 17th century, the boyars of Muscovy formed a closed aristocratic class that surrounded the throne of the grand prince (later the tsar) and ruled the country together with him. They were drawn from about 200 families, descended from former princes, old Moscow (Moscow, Grand Principality of) boyar families, and foreign aristocrats. The rank of boyar did not belong to all members of these families but only to those senior members to whom the tsar granted this title. Below the boyars stood the group of okolnichy. Together these two strata formed the boyar council, which helped the tsar direct the internal and foreign affairs of the state. The decisions of the boyar council, as confirmed by the tsar, were recognized as the normal form of legislation. The boyars and okolnichy generally served as heads of government offices, provincial governors, and military commanders.

The tsar did not have complete freedom in the choice of his chief aides and subordinates. He was bound by the peculiar aristocratic custom of mestnichestvo. This was a complicated hierarchy of precedence among aristocratic Muscovite families. They were ranked in a definite genealogical order according to their relative seniority, and, in the course of filling the highest posts in his army and administration, the tsar had to consider not so much the candidate's personal merits as his genealogical seniority as defined by earlier precedents. Mestnichestvo, which hampered the selection of appropriate candidates for high offices, caused endless quarrels among the boyar families and was finally abolished in 1682.

Throughout the 17th century, the social and political importance of the boyars declined. Early in the 18th century, Tsar Peter I the Great abolished the rank and title of boyar and made state service the exclusive means of attaining a high position in the bureaucratic hierarchy.