The Industrial Workers of the World (IWW), nicknamed "Wobblies", is an international labor union founded in Chicago in 1905. The nickname's origin is uncertain. Its ideology combines general unionism with industrial unionism, as it is a general union, subdivided between the various industries which employ its members. The philosophy and tactics of the IWW are described as "revolutionary industrial unionism", with ties to socialist, syndicalist, and anarchist labor movements.

In the 1910s and early 1920s, the IWW achieved many of its short-term goals, particularly in the American West, and cut across traditional guild and union lines to organize workers in a variety of trades and industries. At their peak in August 1917, IWW membership was estimated at more than 150,000, with active wings in the United States, the UK, Canada, and Australia. The extremely high rate of IWW membership turnover during this era (estimated at 133% per decade) makes it difficult for historians to state membership totals with any certainty, as workers tended to join the IWW in large numbers for relatively short periods (e.g., during labor strikes and periods of generalized economic distress).

Membership declined dramatically in the late 1910s and 1920s. There were conflicts with other labor groups, particularly the American Federation of Labor (AFL), which regarded the IWW as too radical, while the IWW regarded the AFL as too conservative and opposed their decision to divide workers on the basis of their crafts. Membership also declined due to government crackdowns on radical, anarchist, and socialist groups during the First Red Scare after World War I. In Canada the IWW was outlawed by the federal government by an Order in Council on September 24, 1918.

Probably the most decisive factor in the decline in IWW membership and influence was a 1924 schism in the organization, from which the IWW never fully recovered.

The IWW promotes the concept of "One Big Union", and contends that all workers should be united as a social class to supplant capitalism and wage labor with industrial democracy. It is known for the Wobbly Shop model of workplace democracy, in which workers elect their own managers and other forms of grassroots democracy (self-management) are implemented. The IWW does not require its members to work in a represented workplace, neither does it exclude membership in another labor union.