Far-left politics are politics further to the left on the political spectrum than the standard political left. In certain instances—especially in the news media—far left has been associated with various forms of authoritarianism, anarchism, communism, and Marxism, or are characterised as groups that advocate for revolutionary socialism and related communist ideologies, or anti-capitalism and anti-globalisation. Far-left terrorism consists of extremist, militant, or insurgent groups that attempt to realise their ideals through political violence rather than using democratic processes.

Far-left politics are the leftmost ideologies on the left of the left–right political spectrum. They are a heterogeneous group of ideologies within left-wing politics, and wide variety exists between different far-left groups. Ideologies such as communism and anarchism are typically described as far-left. Far-left politics is typically regarded as being to the left of social democracy. As with all political alignments, the exact boundaries of centre-left versus far-left within left-wing politics are not clearly defined and can vary depending on context. Far-left parties sometimes avoid anti-capitalist rhetoric to appeal to the centre-left, while the centre-left may invoke language of radicalism or incorporate some of its ideas. The modern far-left distinguishes itself from social democracy through its inherent opposition to capitalism, neoliberalism, and globalisation.

Academic study of far-left politics often uses radical left as an all-encompassing term, though some far-left groups object to this usage as derogatory. Extreme left and anti-capitalist are also commonly used as synonyms for the far-left. The radical left and the extreme left are often used as equivalents, though some writers create distinctions between them. Hard left may also be used. Far-left political parties use a variety of descriptors for themselves, including workers', labour, socialist, communist, militant, and revolutionary parties. Far-left ideologies are typically derived from either anarchism or Marxism, and the two are frequently combined or integrated together.

A unified working class has traditionally been the focus of far-left movements. Karl Marx defined the working class in the 19th century to include all waged employees of all industries. The development of middle management and decline of the petite bourgeoisie complicated the definition over time. The modern European far-left overall has higher educational attainment than its far-right equivalent. Students and intellectuals have often been inclined to support far-left politics. The far-left may appeal to independent producers or craftsmen who fear competition for large corporations. The unemployed, including the elderly and disabled, are associated with the working class as defined by the far-left because of the disadvantages they may face. The majority of left-leaning labourers preferred social democracy over far-left ideologies.

Communism is the belief that humanity should abandon class divisions in favour of a communist society organised around the needs and abilities of its citizens. Modern communism is a form of revolutionary socialism based on support for the communist society described in the writing of Karl Marx and Friedrich Engels, known as Marxism. Since its introduction, elements of Marxism have become a common factor between most far-left groups. Communist ideology appears in a variety of forms, especially since the dissolution of the Soviet Union left communists without a unifying force.

Marxism specifically opposes capitalism, in contrast to anarchists who oppose both capitalism and the state in its entirety. Marxism sees the proletariat as the primary movers of the revolution. Marx and Engels observed that the working class of 19th century Europe had little influence on or attachment to their nations, and they espoused socialist internationalism that framed class struggle as an issue that united workers without regard to national affiliation. They considered socialist movements within a nation to be necessary only as a means to challenge that nation's bourgeoisie. Marx and Engels believed that the most developed nations were the most likely to see a communist revolution.

Leninists, followers of Russian Marxist Vladimir Lenin, believe that capitalism should be replaced by a dictatorship of the proletariat, which would cause capitalism to degrade and quickly disappear. Leninist-led revolutions have imposed authoritarian rule over society despite invoking concepts of equality when seeking power. The Bolshevik revolution created soviet councils that were to serve as a democratic method of achieving the dictatorship of the proletariat. With the failure of revolutions in Western Europe, Leninism moved away from the belief that revolutions would occur in more developed nations through popular unrest. According to Luke March, the ideology was unsuccessful in more developed nations, as the middle and upper classes were more established, and the unionist working class outnumbered any revolutionary peasantry.

Marxism–Leninism has historically been a major far-left ideology, especially before the dissolution of the Soviet Union. Stalinism supports a one-party state with a planned economy. Stalin abandoned socialist internationalism and established his policy of socialism in one country, where the Communist Party of the Soviet Union prioritised its own stability. It abandoned efforts to spread communism to Western Europe, and it exercised control over the Communist parties of neighbouring countries to reinforce its own strength.