In 1945, union membership reached a peak of 33.4 percent of the U.S. workforce but steadily declined to a low of 10.7 percent in 2017, according to the Bureau of Labor Statistics. This doesn’t include the 1.2 percent of workers who are not affiliated with a union but worked in occupations that were under a union contract. Union membership was at its highest when manufacturing jobs in the U.S. were dominant. The loss of millions of those jobs is one of the major reasons for the decline of union membership. The membership numbers for the top 10 labor unions are self-reported, and unless specifically noted, the unions are headquartered in Washington, D.C.

National Education Association

With more than 3 million members, the NEA represents classroom teachers, education support professionals, higher education staff and faculty, retired teachers and students training to become education professionals. Founded in 1857, the NEA merged with the American Teachers Association in 1966.

Service Employees International Union

The SEIU has nearly 1.9 million members in more than 100 occupations in the U.S., including janitors, security workers, superintendents, maintenance workers, local and state government employees, bus drivers and child-care providers. Founded by janitorial workers in 1921, the SEIU is now the largest-membership union in the U.S. that represents the property services, public services and health-care industries.

American Federation of Teachers

The AFT was founded in Chicago in 1916, and now has more than 1.5 million members, including pre-K through grade 12 teachers, early childhood educators, paraprofessionals and other school-related personnel, higher education faculty and staff, government employees and other education professionals.

International Brotherhood of Teamsters

In 1903, two unions merged to form the Teamsters, one of the most recognized unions in the U.S. It has 1.4 million members in 21 industrial divisions, including airlines, bakery and laundry, brewery and soft drinks, building material and construction, entertainment and passenger transportation.

American Federation of State, County and Municipal Employees

With an estimated 1.3 million members, the AFSCME is the biggest public services employee union in the U.S., representing corrections officers, childcare providers, sanitation workers, EMTs and nurses. The union was founded in 1932 by a group of state employees in Madison, Wisconsin.

United Food and Commercial Workers International Union

The UFCW was founded in 1979 and has grown to 1.3 million members in occupations such as food-processing workers, drugstore workers, poultry processing plant workers, packinghouse employees and grocery store workers.

United Steelworkers

The USW was founded in 1942 and is headquartered in Pittsburgh. It has more than 1.2 million members in the steel, aluminum and metal working industry, and also represents chemical plant workers, pharmacy workers, rubber workers and construction workers.

The International Union, United Automobile, Aerospace and Agricultural Implement Workers of America

This union is more commonly referred to as the United Auto Workers and has more than 400,000 active members, and more than 580,000 retired members in the U.S., Canada and Puerto Rico. The UAW, which was founded in 1935 and is headquartered in Detroit, represents autoworkers, health-care workers, academic student employees, postdoctoral scholars and workers in the casino gambling industry.

International Association of Machinists and Aerospace Workers

Founded in 1888 by a group of Atlanta-based machinists, the IAM is headquartered in Upper Marlboro, Maryland, and has 720,000 members. The union represents automotive repair workers, city employees, truck assemblers, fabrication workers and aerospace workers.

The International Brotherhood of Electrical Workers

With 675,000 members in the utilities, construction, telecommunications, broadcasting, manufacturing and railroads industries, the IBEW has been active since it was founded in St. Louis in 1891.