The American Civil Liberties Union (ACLU) is an American nonprofit human rights organization founded in 1920. The organization strives "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 more than 1,800,000 members as of July 2018, with an annual budget over $300 million. ACLU affiliates are active in all 50 states, Washington, D.C., and Puerto Rico. The ACLU provides legal assistance in cases where it considers civil liberties 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.

Civil liberties are guarantees and freedoms that governments commit not to abridge, either by constitution, legislation, or judicial interpretation, without due process. Though the scope of the term differs between countries, civil liberties may include the freedom of conscience, freedom of press, freedom of religion, freedom of expression, freedom of assembly, the right to security and liberty, freedom of speech, the right to privacy, the right to equal treatment under the law and due process, the right to a fair trial, and the right to life. Other civil liberties include the right to own property, the right to defend oneself, and the right to bodily integrity. Within the distinctions between civil liberties and other types of liberty, distinctions exist between positive liberty/positive rights and negative liberty/negative rights.