An amicus curiae (also amicus curiæ; plural amici curiae, literally "friend of the court") is someone who is not a party to a case who offers information that bears on the case but that has not been solicited by any of the parties to assist a court. This may take the form of legal opinion, testimony or learned treatise (the amicus brief) and is a way to introduce concerns ensuring that the possibly broad legal effects of a court decision will not depend solely on the parties directly involved in the case. The decision on whether to admit the information lies at the discretion of the court. The phrase amicus curiae is legal Latin.
The amicus curiae figure originates in Roman law. Starting in the 9th century, it was incorporated to English law, and it was later extended to most common law systems. Later, it was also introduced in international law, in particular concerning human rights. From there, it was integrated in some civil law systems (it has recently been integrated into Argentina's law system and Honduras's 2010 civil procedures code). Today, it is used by the European Court of Human Rights, the Inter-American Commission on Human Rights, the Inter-American Court of Human Rights, and the Court of Justice of the European Union.
The role of an amicus is often confused with that of an intervener. The role of an amicus is, as stated by Salmon LJ (as Lord Salmon then was) in Allen v Sir Alfred McAlpine & Sons Ltd  2 QB 229 at p. 266 F-G:I had always understood that the role of an amicus curiae was to help the court by expounding the law impartially, or if one of the parties were unrepresented, by advancing the legal arguments on his behalf.
The situation most often noted in the press is when an advocacy group files a brief in a case before an appellate court to which it is not a litigant. Appellate cases are normally limited to the factual record and arguments coming from the lower court case under appeal; attorneys focus on the facts and arguments most favorable to their clients. Where a case may have broader implications, amicus curiae briefs are a way to introduce those concerns, so that the possibly broad legal effects of court decisions will not depend solely on the parties directly involved in the case.
In prominent cases, amici curiae are generally organizations with sizable legal budgets. In the United States, for example, non-profit legal advocacy organizations, such as the American Civil Liberties Union, the Landmark Legal Foundation, the Pacific Legal Foundation, the Electronic Frontier Foundation, the American Center for Law and Justice or NORML, frequently submit such briefs to advocate for or against a particular legal change or interpretation. If a decision could affect an entire industry, companies other than the litigants may wish to have their concerns heard. In the United States, federal courts often hear cases involving the constitutionality of state laws. Hence states may file briefs as amici curiae when their laws are likely to be affected, as in the Supreme Court case McDonald v. Chicago, when thirty-two states under the aegis of Texas (and California independently) filed such briefs.
Amici curiae who do not file briefs often present an academic perspective on the case. For example, if the law gives deference to a history of legislation of a certain topic, a historian may choose to evaluate the claim from specialized expertise. An economist, statistician, or sociologist may choose to do the same. Newspaper editorials, blogs, and other opinion pieces arguably have the capability to influence Supreme Court decisions as de facto amici curiae. They are not, however, technically considered amicus curiae, as they do not submit materials to the Court, do not need to ask for leave, and have no guarantee that they will be read.
The Supreme Court of the United States has special rules for amicus curiae briefs sought to be filed in cases pending before it. Supreme Court Rule 37 states, in part, such a brief should cover "relevant matter" not dealt with by the parties which "may be of considerable help". The cover of an amicus brief must identify which party the brief is supporting, or if the brief supports only affirmance or reversal. Supreme Court Rule 37.3(a). The Court also requires that, inter alia, all non-governmental amici identify those providing a monetary contribution to the preparation or submission of the brief. Supreme Court Rule 37.6. Briefs must be prepared in booklet format, and 40 copies must be served with the Court.
In the United States Supreme Court, unless the amicus brief is being filed by the federal government (or one of its officers or agents) or a U.S. state, permission of the court (by means of motion for leave) or mutual consent of the parties is generally required. Allowing an amicus curiae to present oral argument is considered "extraordinary".
Justice, To Assist the Court: Third Party Interventions in the UK (2009)