Mikhail Sergeyevich Gorbachev, who was the last leader of the Soviet Union and oversaw its dissolution, died Tuesday at a hospital in Moscow, according to Russia’s state news agency TASS. He was 91 years old.

In context: Gorbachev played a pivotal role in the dissolution of the Soviet Union and the formation of Russia as a modern state in 1991. He is known for his glasnost and perestroika policies, which championed government transparency and economic liberalization. In 1990 the Nobel Committee awarded him a Nobel Peace Prize for helping end the Cold War.

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Gorbachev died “after a serious and prolonged illness,” according to the hospital, TASS reported. He will be buried next to his wife Raisa at the Novo-Dyevitchiye cemetery in Moscow.