Anarcho-syndicalism is an anarchist organisational model that centres trade unions as a vehicle for class conflict. Drawing from the theory of libertarian socialism and the practice of syndicalism, anarcho-syndicalism sees trade unions as both a means to achieve immediate improvements to working conditions and to build towards a social revolution in the form of a general strike, with the ultimate aim of abolishing the state and capitalism. Anarcho-syndicalists consider trade unions to be the prefiguration of a post-capitalist society and seek to use them in order to establish workers' control of production and distribution. An anti-political ideology, anarcho-syndicalism rejects political parties and participation in parliamentary politics, considering them to be a corrupting influence on the labour movement. In order to achieve their material and economic goals, anarcho-syndicalists instead practice direct action in the form of strike actions, boycotts and sabotage. Anarcho-syndicalists also attempt to build solidarity among the working class, in order to unite workers against the exploitation of labour and build workers' self-management.

The foundations of anarcho-syndicalism were laid by the anti-authoritarian faction of the International Workingmen's Association (IWMA) and developed by the French General Confederation of Labour (CGT). Anarcho-syndicalism was constituted as a specific tendency following the International Anarchist Congress of Amsterdam in 1907, which led to anarcho-syndicalism becoming the dominant form of trade union organisation in Europe and Latin America. After facing suppression during the Revolutions of 1917–1923, anarcho-syndicalists established the International Workers' Association (IWA) in 1922. Anarcho-syndicalism reached its apex during the Spanish Revolution of 1936, when the National Confederation of Labour (CNT) established an anarcho-syndicalist economy throughout much of the Spanish Republic. Anarcho-syndicalism went into decline after the defeat of the anarchists in the Spanish Civil War in 1939. The movement split into two factions: the "orthodox" faction, which held to traditional syndicalist principles in spite of changing material conditions; and the "revisionist" faction, which aimed to achieve a mass base and work within the framework of newly-established welfare states. By the end of the 20th century, the rise of neoliberalism and the collapse of the Eastern Bloc had led to a revival in anarcho-syndicalism, with syndicalist unions once again being established throughout the globe.

The history of anarcho-syndicalism can be traced back to the anarchist faction of the International Workingmen's Association (IWA), which called for trade unions to overthrow the state in a general strike. This syndicalist model of trade union organisation was adopted by anarchists in Spain, Cuba, Mexico and the United States, where syndicalism became the dominant organisational form. Elsewhere, the development of reformist tendencies such as social democracy sidelined anarchists within trade unions. But tensions between rank-and-file trade unionists and their social-democratic leadership eventually gave way to the development of revolutionary syndicalism, which called for workers themselves to take direct action in order to improve their own material conditions.

Revolutionary syndicalism was first propagated in France, where the Bourses du Travail (English: Labour Exchanges) were established to provide mutual aid to workers and organise strike actions. By the 20th century, the bourses had joined together to establish the General Confederation of Labour (CGT), which rose to include 60% of French workers within its ranks. But after the CGT launched a general strike, which won French workers the eight-hour day and the weekend, the union turned away from revolutionary syndicalism towards reformism. However, by this time, revolutionary syndicalism had already spread throughout Europe, with syndicalist unions being established in the Netherlands, Italy, Portugal, Germany and Sweden. The concurrent development of industrial unionism led to the establishment of the Industrial Workers of the World (IWW) in countries throughout the English-speaking world. By the 1910s, syndicalism had spread throughout every country in Europe and anarchist tendencies started to develop within the movement.

Although anarchists widely participated in the syndicalist movement, syndicalists were divided into separate tendencies, many of which rejected the ideological prescriptions of anarchism. At the International Anarchist Congress of Amsterdam, convened by the anarcho-syndicalist Christiaan Cornelissen in 1907, a conflict between the two tendencies broke out; while the syndicalist Pierre Monatte and the anarchist Errico Malatesta debated their respective ideologies, Amédée Dunois attempted to synthesise the two into a "workers' anarchism". Anarcho-syndicalism soon became the dominant form of trade union organisation in Spain, Argentina, Brazil, Mexico, Paraguay, Uruguay and many other Latin American countries, where anarcho-syndicalists organised a series of general strikes for the eight-hour day. Attempts to establish international links between these anarcho-syndicalist organisations culminated in the convocation of a revolutionary syndicalist congress in 1913, although further movements towards the creation of an international organisation were halted by the outbreak of world war.

The outbreak of World War I split anarcho-syndicalists into internationalist and defencist camps; the former declared themselves against both sides of the war, while the latter supported the Allies against the Central Powers. The conflict ultimately demonstrated the inability of the international syndicalist movement to prevent war and discredited "neutral syndicalism" in the eyes of many workers, causing revolutionary sentiments to once again begin rising within the workers' movement. When the Revolutions of 1917–1923 spread throughout Europe, anarcho-syndicalists became keen participants in the revolutionary wave, during which they faced rising political repression. Anarcho-syndicalists in Europe, Latin America and Asia organised general strikes, sometimes reaching revolutionary proportions, but were ultimately suppressed by nationalist or communist dictatorships.

Following the suppression of their movements and the rising influence of Marxism-Leninism throughout the world, anarcho-syndicalists moved to establish their own international organisation: the International Workers' Association (IWA). Theoretical discussions within the IWA led to debates on the issues of platformism, insurrectionism, industrialism and reformism, while the Spanish National Confederation of Labour (CNT) rose to prominence in the Spanish Republic. With the outbreak of the Spanish Civil War, the CNT led the defeat of the Nationalists in Catalonia, where they ignited an anarcho-syndicalist revolution that collectivised three-quarters of the Republican economy. The CNT's decision to join the Republican government caused controversy within the IWA and the social revolution was eventually suppressed by the government. The Republicans ultimately lost the war and the Spanish anarcho-syndicalists were imprisoned, killed or forced into exile by the victorious Francoist dictatorship.

During World War II, anarcho-syndicalists initially adopted an "internationalist" position, taking up the slogan "Neither Fascism, nor Antifascism." But following the Nazi occupation of Europe and the subsequent suppression of their organisations, anarcho-syndicalists throughout Europe reorientated themselves towards anti-fascism and joined the anti-fascist resistance. After the war, anarcho-syndicalism experienced a rapid decline, as anarcho-syndicalist unions were either marginalised by rising social corporatism or repressed by newly-established authoritarian states. Anarcho-syndicalists proved unable to keep up with the changes in the post-war capitalist system, which hastened the decline of the anarcho-syndicalist movement and forced its organisations to choose between marginalisation, reform or dissolution. Despite the economic changes, the IWA chose to reaffirm traditional anarcho-syndicalist principles, causing its Dutch and Swedish sections to split from it. The SAC chose to revise its principles in order to adapt to the new Swedish welfare state, within which they aimed to establish workers' control over welfare and the democratisation of the economy. The IWA declined to its lowest point during the 1960s, as its membership became increasingly preoccupied with theory.