The Congress of Industrial Organizations (CIO) was a federation of unions that organized workers in industrial unions in the United States and Canada from 1935 to 1955. Originally created in 1935 as a committee within the American Federation of Labor (AFL) by John L. Lewis, a member of the United Mine Workers (UMW), and called the Committee for Industrial Organization, its name was changed in 1938 when it broke away from the American Federation of Labor. It also changed names because it was not successful with organizing unskilled workers within the AFL.The CIO supported Franklin D. Roosevelt and the New Deal Coalition, and was open to African Americans. Both the CIO and its rival the AFL grew rapidly during the Great Depression. The rivalry for dominance was bitter and sometimes violent. The Congress for Industrial Organization was founded on November 9, 1935, by eight international unions belonging to the American Federation of Labor.

In its statement of purpose, the CIO said it had formed to encourage the AFL to organize workers in mass production industries along industrial union lines. The CIO failed to change AFL policy from within. On September 10, 1936, the AFL suspended all 10 CIO unions (two more had joined in the previous year). In 1938, these unions formed the Congress of Industrial Organizations as a rival labor federation. Section 504 of the Taft–Hartley Act of 1947 required union leaders to swear that they were not Communists, which many CIO leaders refused to do; in 1965, the Supreme Court struck down this part of the law as unconstitutional. In 1955, the CIO rejoined the AFL, forming the new entity known as the American Federation of Labor and Congress of Industrial Organizations (AFL–CIO).