Marxism–Leninism is a communist ideology that was the main communist movement throughout the 20th century. Developed in Russia by the Bolsheviks, it was the state ideology of the Soviet Union, Soviet satellite states in the Eastern Bloc, and various countries in the Non-Aligned Movement and Third World during the Cold War, as well as the Communist International after Bolshevisation. Today, Marxism–Leninism is the ideology of the ruling parties of China, Cuba, Laos and Vietnam (all one-party socialist republics), as well as many other Communist parties. The state ideology of North Korea is derived from Marxism–Leninism (although its evolution is disputed). Marxist–Leninist states are commonly referred to as "communist states" by Western academics. Marxist–Leninists reject anarchism and left communism, as well as reformist socialism and social democracy. They oppose fascism, imperialism, and liberal democracy. Marxism–Leninism holds that a two-stage communist revolution is needed to replace capitalism. A vanguard party, organized through democratic centralism, would seize power on behalf of the proletariat and establish a one-party socialist state, called the dictatorship of the proletariat. The state would control the means of production, suppress opposition, counter-revolution, and the bourgeoisie, and promote Soviet collectivism, to pave the way for an eventual communist society that would be classless and stateless.

Marxism–Leninism was developed from Bolshevism by Joseph Stalin in the 1920s based on his understanding and synthesis of orthodox Marxism and Leninism. After the death of Vladimir Lenin in 1924, Marxism–Leninism became a distinct movement in the Soviet Union when Stalin and his supporters gained control of the party. It rejected the common notion among Western Marxists of world revolution as a prerequisite for building socialism, in favour of the concept of socialism in one country. According to its supporters, the gradual transition from capitalism to socialism was signified by the introduction of the first five-year plan and the 1936 Soviet Constitution. By the late 1920s, Stalin established ideological orthodoxy in the Russian Communist Party (Bolsheviks), the Soviet Union, and the Communist International to establish universal Marxist–Leninist praxis. The formulation of the Soviet version of dialectical and historical materialism in the 1930s by Stalin and his associates, such as in Stalin's text "Dialectical and Historical Materialism", became the official Soviet interpretation of Marxism, and was taken as example by Marxist–Leninists in other countries; according to the Great Russian Encyclopedia, this text became the foundation of the philosophy of Marxism–Leninism. In 1938, Stalin's official textbook History of the Communist Party of the Soviet Union (Bolsheviks) popularised Marxism–Leninism.

The internationalism of Marxism–Leninism was expressed in supporting revolutions in other countries, initially through the Communist International and then through the concept of socialist-leaning countries after de-Stalinisation. The establishment of other Communist states after World War II resulted in Sovietisation, and these states tended to follow the Soviet Marxist–Leninist model of five-year plans and rapid industrialisation, political centralisation, and repression. During the Cold War, Marxism–Leninism was a driving force in international relations. With the death of Stalin and the ensuing de-Stalinisation, Marxism–Leninism underwent several revisions and adaptations such as Guevarism, Ho Chi Minh Thought, Hoxhaism, Maoism, socialism with Chinese characteristics, and Titoism. More recently Nepalese communist parties have adopted People's Multiparty Democracy. This also caused several splits between Marxist–Leninist states, resulting in the Tito–Stalin split, the Sino-Soviet split, and the Sino-Albanian split. The socio-economic nature of Marxist–Leninist states, especially that of the Soviet Union during the Stalin era, has been much debated, varyingly being labelled a form of bureaucratic collectivism, state capitalism, state socialism, or a totally unique mode of production. The Eastern Bloc, including Marxist–Leninist states in Central and Eastern Europe as well as the Third World socialist regimes, have been variously described as "bureaucratic-authoritarian systems", and China's socio-economic structure has been referred to as "nationalistic state capitalism".

Criticism of Marxism–Leninism largely overlaps with criticism of Communist party rule and mainly focuses on the actions and policies of Marxist–Leninist leaders, most notably Stalin and Mao Zedong. Marxist–Leninist states have been marked by a high degree of centralised control by the state and Communist party, political repression, state atheism, collectivisation and use of labour camps, as well as free universal education and healthcare, low unemployment and lower prices for certain goods. Historians such as Silvio Pons and Robert Service stated that the repression and totalitarianism came from Marxist–Leninist ideology. Historians such as Michael Geyer and Sheila Fitzpatrick have offered other explanations and criticise the focus on the upper levels of society and use of concepts such as totalitarianism which have obscured the reality of the system. While the emergence of the Soviet Union as the world's first nominally Communist state led to communism's widespread association with Marxism–Leninism and the Soviet model, several academics say that Marxism–Leninism in practice was a form of state capitalism.