Labor unions represent United States workers in many industries recognized under US labor law since the 1935 enactment of the National Labor Relations Act. Their activity today centers on collective bargaining over wages, benefits, and working conditions for their membership, and on representing their members in disputes with management over violations of contract provisions. Larger trade unions also typically engage in lobbying activities and electioneering at the state and federal level.

Most unions in the United States are aligned with one of two larger umbrella organizations: the AFL–CIO created in 1955, and the Change to Win Federation (current Strategic Organizing Center) (SOC) which split from the AFL–CIO in 2005. Both advocate policies and legislation on behalf of workers in the United States and Canada, and take an active role in politics. The AFL–CIO is especially concerned with global trade issues.

The percentage of workers belonging to a union (or total labor union "density") varies by country. In 2022 it was 10.1% in the United States, compared to 20.1% in 1983. There were 14.3 million members in the U.S. in 2022, down from 17.7 million in 1983. Union membership in the private sector has fallen to 6.0%, one fifth that of public sector workers, at 33.1% (2022). From a global perspective, in 2016 the US had the fifth lowest trade union density of the 36 OECD member nations.

In the 21st century, the most prominent unions are among public sector employees such as city employees, government workers, teachers and police. Members of unions are disproportionately older, male, and residents of the Northeast, the Midwest, and California. Union workers average 10-30% higher pay than non-union in the United States after controlling for individual, job, and labor market characteristics.

Although much smaller compared to their peak membership in the 1950s, American unions remain a political factor, both through mobilization of their own memberships and through coalitions with like-minded activist organizations around issues such as immigrant rights, environmental protections, trade policy, health care, and living wage campaigns. Of special concern are efforts by cities and states to reduce the pension obligations owed to unionized workers who retire in the future. Republicans elected with Tea Party support in 2010, most notably former Governor Scott Walker of Wisconsin, have launched major efforts against public sector unions due in part to state government pension obligations along with the allegation that the unions are too powerful.

The academic literature shows substantial evidence that labor unions reduce economic inequality. Research suggests that rising income inequality in the United States is partially attributable to the decline of the labor movement and union membership,: 1  and that this is not only a correlation. Research has also found that unions can harm profitability, employment and business growth rates.

Unions began forming in the mid-19th century in response to the social and economic impact of the Industrial Revolution. National labor unions began to form in the post-Civil War Era. The Knights of Labor emerged as a major force in the late 1880s, but it collapsed because of poor organization, lack of effective leadership, disagreement over goals, and strong opposition from employers and government forces.

The American Federation of Labor, founded in 1886 and led by Samuel Gompers until his death in 1924, proved much more durable. It arose as a loose coalition of various local unions. It helped coordinate and support strikes and eventually became a major player in national politics, usually on the side of the Democrats.

American labor unions benefited greatly from the New Deal policies of Franklin Delano Roosevelt in the 1930s. The Wagner Act, in particular, legally protected the right of unions to organize. Unions from this point developed increasingly closer ties to the Democratic Party, and are considered a backbone element of the New Deal Coalition.

Pro-business conservatives gained control of Congress in 1946, and in 1947 passed the Taft–Hartley Act, drafted by Senator Robert A. Taft. President Truman vetoed it but the Conservative coalition overrode the veto. The veto override had considerable Democratic support, including 106 out of 177 Democrats in the House, and 20 out of 42 Democrats in the Senate. The law, which is still in effect, banned union contributions to political candidates, restricted the power of unions to call strikes that "threatened national security," and forced the expulsion of Communist union leaders (the Supreme Court found the anti-communist provision to be unconstitutional, and it is no longer in force). The unions campaigned vigorously for years to repeal the law but failed. During the late 1950s, the Landrum Griffin Act of 1959 passed in the wake of Congressional investigations of corruption and undemocratic internal politics in the Teamsters and other unions.

In 1955, the two largest labor organizations, the AFL and CIO, merged, ending a division of over 20 years. AFL President George Meany became President of the new AFL–CIO, and AFL Secretary-Treasurer William Schnitzler became AFL–CIO Secretary-Treasurer. The draft constitution was primarily written by AFL Vice President Matthew Woll and CIO General Counsel Arthur Goldberg, while the joint policy statements were written by Woll, CIO Secretary-Treasurer James Carey, CIO vice presidents David McDonald and Joseph Curran, Brotherhood of Railway Clerks President George Harrison, and Illinois AFL–CIO President Reuben Soderstrom.

The percentage of workers belonging to a union (or "density") in the United States peaked in 1954 at almost 35% and the total number of union members peaked in 1979 at an estimated 21.0 million. Membership has declined since, with private sector union membership beginning a steady decline that continues into the 2010s, but the membership of public sector unions grew steadily.

After 1960 public sector unions grew rapidly and secured good wages and high pensions for their members. While manufacturing and farming steadily declined, state- and local-government employment quadrupled from 4 million workers in 1950 to 12 million in 1976 and 16.6 million in 2009. Adding in the 3.7 million federal civilian employees, in 2010 8.4 million government workers were represented by unions, including 31% of federal workers, 35% of state workers and 46% of local workers.

By the 1970s, a rapidly increasing flow of imports (such as automobiles, steel and electronics from Germany and Japan, and clothing and shoes from Asia) undercut American producers. By the 1980s there was a large-scale shift in employment with fewer workers in high-wage sectors and more in the low-wage sectors. Many companies closed or moved factories to Southern states (where unions were weak), countered the threat of a strike by threatening to close or move a plant, or moved their factories offshore to low-wage countries. The number of major strikes and lockouts fell by 97% from 381 in 1970 to 187 in 1980 to only 11 in 2010. On the political front, the shrinking unions lost influence in the Democratic Party, and pro-Union liberal Republicans faded away. Union membership among workers in private industry shrank dramatically, though after 1970 there was growth in employees unions of federal, state and local governments. The intellectual mood in the 1970s and 1980s favored deregulation and free competition. Numerous industries were deregulated, including airlines, trucking, railroads and telephones, over the objections of the unions involved. The climax came when President Ronald Reagan—a former union president—broke the illegal Professional Air Traffic Controllers Organization (PATCO) strike in 1981, dealing a major blow to unions.