Pugachev's Rebellion
Pugachev's Rebellion (Peasants' War 1773-75, Cossack Rebellion) of 1773-75 was the principal revolt in a series of popular rebellions that took place in the Russian Empire after Catherine II seized power in 1762. It began as an organized insurrection of Yaik Cossacks headed by Yemelyan Pugachev, a disaffected ex-lieutenant of the Imperial Russian Army, against a background of profound peasant unrest and war with the Ottoman Empire. After initial success, Pugachev assumed leadership of an alternative government in the name of the assassinated Tsar Peter III and proclaimed an end to serfdom. This organized leadership presented a challenge to the imperial administration of Catherine II.
Bulavin Rebellion
The Bulavin Rebellion (Astrakhan Revolt) is the name given to a war 
of Don Cossacks against Imperial Russia between the years 1707 and 1708. The war was led by Kondraty Bulavin, a democratically elected Ataman of Don Cossacks. The war was triggered by a number of underlying tensions between the Imperial government under Peter I of Russia, the Cossacks, and Russian peasants fleeing from serfdom in Russia to gain freedom in the autonomous Don area. It started with the assassination of Prince Yury Dolgorukov, the leader of Imperial army's punitive expedition to the Don area, by Don Cossacks under Bulavin's command. The end of the rebellion is associated with Bulavin's death in 1708.