The October Revolution, officially known as the Great October Socialist Revolution under the Soviet Union, also known as the Bolshevik Revolution, was a revolution in Russia led by the Bolshevik Party of Vladimir Lenin that was a key moment in the larger Russian Revolution of 19171923. It was the second revolutionary change of government in Russia in 1917. It took place through an armed insurrection in Petrograd (now Saint Petersburg) on 7 November 1917 [O.S. 25 October]. It was the precipitating event of the Russian Civil War.

The October Revolution followed and capitalized on the February Revolution earlier that year, which had overthrown the Tsarist autocracy, resulting in a liberal provisional government. The provisional government had taken power after being proclaimed by Grand Duke Michael, Tsar Nicholas II's younger brother, who declined to take power after the Tsar stepped down. During this time, urban workers began to organize into councils (soviets) wherein revolutionaries criticized the provisional government and its actions. The provisional government remained unpopular, especially because it was continuing to fight in World War I, and had ruled with an iron fist throughout the summer (including killing hundreds of protesters in the July Days).

Events came to a head in the fall as the Directorate, led by the left-wing Socialist Revolutionary Party, controlled the government. The left-wing Bolsheviks were deeply unhappy with the government, and began spreading calls for a military uprising. On 10 October 1917 (O.S.; 23 October, N.S.), the Petrograd Soviet, led by Trotsky, voted to back a military uprising. On 24 October (O.S.; 6 November, N.S.) the government shut down numerous newspapers and closed the city of Petrograd in an attempt to forestall the revolution; minor armed skirmishes broke out. The next day a full scale uprising erupted as a fleet of Bolshevik sailors entered the harbor and tens of thousands of soldiers rose up in support of the Bolsheviks. Bolshevik Red Guards forces under the Military-Revolutionary Committee began the occupation of government buildings on 25 October (O.S.; 7 November, N.S.), 1917. The following day, the Winter Palace (the seat of the Provisional government located in Petrograd, then capital of Russia) was captured.

As the Revolution was not universally recognized, the country descended into the Russian Civil War, which would last until 1923 and ultimately lead to the creation of the Soviet Union in late 1922. The historiography of the event has varied. The victorious Soviet Union viewed it as a validation of their ideology, and the triumph of the worker over capitalism. During Soviet times, revolution day was a national holiday, marking its importance in the country's founding story. On the other hand, the Western Allies saw it as a totalitarian coup, which used the democratic Soviet councils only until they were no longer useful. The event inspired many cultural works, and ignited communist movements across Europe and globally. Many MarxistLeninist parties around the world still celebrate October Revolution Day.

Vladimir Ilyich Ulyanov (22 April [O.S. 10 April] 1870 – 21 January 1924), better known by his alias Lenin, was a Russian revolutionary, politician, and political theorist. He served as the first and founding head of government of Soviet Russia from 1917 to 1924 and of the Soviet Union from 1922 to 1924. Under his administration, Russia, and later the Soviet Union, became a one-party socialist state governed by the Communist Party. Ideologically a Marxist, he developed a subset of Marxism called Leninism.

Born to an upper middle-class family in Simbirsk, Lenin embraced revolutionary socialist politics following his brother's 1887 execution. Expelled from Kazan Imperial University for participating in protests against the Russian Empire's Tsarist government, he devoted the following years to a law degree. He moved to Saint Petersburg in 1893 and became a senior Marxist activist. In 1897, he was arrested for sedition and exiled to Shushenskoye in Siberia for three years, where he married Nadezhda Krupskaya. After his exile, he moved to Western Europe, where he became a prominent theorist in the Marxist Russian Social Democratic Labour Party (RSDLP). In 1903, he took a key role in the RSDLP ideological split, leading the Bolshevik faction against Julius Martov's Mensheviks. Following Russia's failed Revolution of 1905, he campaigned for the First World War to be transformed into a Europe-wide proletarian revolution, which, as a Marxist, he believed would cause the overthrow of capitalism and its replacement with socialism. After the 1917 February Revolution ousted the Tsar and established a Provisional Government, he returned to Russia to play a leading role in the October Revolution in which the Bolsheviks overthrew the new regime.

Lenin's Bolshevik government initially shared power with the Left Socialist Revolutionaries, elected soviets, and a multi-party Constituent Assembly, although by 1918 it had centralised power in the new Communist Party. Lenin's administration redistributed land among the peasantry and nationalised banks and large-scale industry. It withdrew from the First World War by signing a treaty conceding territory to the Central Powers, and promoted world revolution through the Communist International. Opponents were suppressed in the Red Terror, a violent campaign administered by the state security services; tens of thousands were killed or interned in concentration camps. His administration defeated right and left-wing anti-Bolshevik armies in the Russian Civil War from 1917 to 1922 and oversaw the Polish–Soviet War of 1919–1921. Responding to wartime devastation, famine, and popular uprisings, in 1921 Lenin encouraged economic growth through the New Economic Policy. Several non-Russian nations had secured independence from the Russian Empire after 1917, but three were re-united into the new Soviet Union in 1922. His health failing, Lenin died in Gorki, with Joseph Stalin succeeding him as the pre-eminent figure in the Soviet government.

Widely considered one of the most significant and influential figures of the 20th century, Lenin was the posthumous subject of a pervasive personality cult within the Soviet Union until its dissolution in 1991. He became an ideological figurehead behind Marxism–Leninism and a prominent influence over the international communist movement. A controversial and highly divisive historical figure, Lenin is viewed by his supporters as a champion of socialism and the working class. Meanwhile Lenin's critics accuse him of establishing a totalitarian dictatorship which oversaw mass killings and political repression.