The American Civil Liberties Union (ACLU) is a nonprofit organization founded in 1920 "to defend and preserve the individual rights and liberties guaranteed to every person in this country by the Constitution and laws of the United States". The ACLU works through litigation and lobbying, and has over 1,800,000 members as of July 2018, with an annual budget of over $300 million. Affiliates of the ACLU are active in all 50 states, the District of Columbia, and Puerto Rico. The ACLU provides legal assistance in cases where it considers civil liberties to be at risk. Legal support from the ACLU can take the form of direct legal representation or preparation of amicus curiae briefs expressing legal arguments when another law firm is already providing representation.

In addition to representing persons and organizations in lawsuits, the ACLU lobbies for policy positions that have been established by its board of directors. Current positions of the ACLU include opposing the death penalty; supporting same-sex marriage and the right of LGBT people to adopt; supporting reproductive rights such as birth control and abortion rights; eliminating discrimination against women, minorities, and LGBT people; decarceration in the United States; supporting the rights of prisoners and opposing torture; and upholding the separation of church and state by opposing government preference for religion over non-religion or for particular faiths over others.

Legally, the ACLU consists of two separate but closely affiliated nonprofit organizations, namely the American Civil Liberties Union, a 501(c)(4) social welfare group; and the ACLU Foundation, a 501(c)(3) public charity. Both organizations engage in civil rights litigation, advocacy, and education, but only donations to the 501(c)(3) foundation are tax deductible, and only the 501(c)(4) group can engage in unlimited political lobbying. The two organizations share office space and employees.