The Cossack Hetmanate or Zaporizhian Sich is a political, social and military organization of Ukrainian Cossacks from the 16th to the 18th century. It was established in central Ukraine on the banks of the river Dniepr. Its appearance challenged the authority of the Russian Empire.

The Cossack Hetmanate or Zaporizhian Sich was led by a hetman and the supreme government body called the Cossack Rada. The most famous hetmans are Bohdan Khmelnytsky, Petro Sahaidachny, Pylyp Orlyk, and Ivan Mazepa.

Cossacks fought for their independence from Poland and often began uprisings against the Tsar. However, in later times Cossacks became some of the most loyal troops in the Empire until the revolution, when most Cossacks sided with the Red Army.

After the Treaty of Pereyaslav the hetmanate was at least nominally a part of the Russian Empire although for a long time it enjoyed nearly complete autonomy. The society has been semi-militarized. The Cossacks spread throughout the Russian Empire, and were encouraged to settle on the frontiers. They formed seven voiskos (Don, Semirechinsk, Terek, Zabaikalskae, Zaporozhskoe, Kuban', Ural).

It is not fair to regard Zaporizhian Cossacks as drunken rebels and anarchists. At the turn of the 18th century Zaporizhian Sich was ahead of number of Eastern and Western European countries in terms of literacy. Schools were to be found in most villages. Ukrainian Cossacks were known as well-educated people. Hetmans Bohdan Khmelnytsky, Petro Sahaidachny, and Ivan Mazepa knew several languages.