What Are Amicus Briefs and How Do Courts Use Them?

An amicus brief, short for amicus curiae or “friend of the court,” is a legal filing submitted by someone who is not a party to a lawsuit but wants to help the court understand the broader stakes of its decision. The filer has no direct legal interest in winning or losing. Their role is advisory: they bring information, data, or legal arguments the parties may not have reason to raise. These filings appear in nearly every argued case at the U.S. Supreme Court, which now averages around 16 amicus briefs per case.

What the Brief Is For

Parties in a lawsuit focus on their own interests. An amicus fills the gaps. A medical association might explain how a regulatory ruling would affect clinical practice. A group of economists might submit data showing the financial consequences of a particular reading of antitrust law. A coalition of states might weigh in on a federal case that would reshape how they enforce their own statutes.

These briefs appear most often at the appellate level, especially at the U.S. Supreme Court, but they can also appear in trial courts in high-profile litigation or cases involving unsettled legal questions. What distinguishes an amicus from a party is simple. The amicus isn’t suing anyone and nobody is suing them. They’re there to help the court think.

Who Files Amicus Briefs

The range of filers is broad. Advocacy organizations file regularly across the ideological spectrum, including the American Civil Liberties Union, the NAACP Legal Defense Fund, and the Cato Institute. Corporations and trade associations file when a case could reshape regulation in their industry. Academic institutions and individual scholars contribute empirical research and specialized expertise.

Government filers carry particular weight. The U.S. Solicitor General files on behalf of the federal government so often that the office is sometimes called the “tenth justice.” State attorneys general file frequently in Supreme Court cases, often in multi-state coalitions. Professional organizations like the American Medical Association or the American Bar Association file when cases touch their members’ work.¹ Individual citizens can file too, though without institutional backing or clear expertise, courts are less likely to grant them permission.

How Much Do They Actually Influence Courts?

The volume alone shows how seriously the practice is taken. Between 1946 and 1955, amicus briefs appeared in only about 23 percent of argued Supreme Court cases. By the mid-1980s, that figure had climbed to roughly 85 percent. Since 2010, amicus briefs have been filed in 96 percent of all argued cases. The average per case rose from fewer than one in the late 1940s to about five by the early 1990s to 16 by the 2019-2020 term.

Justices cite amicus briefs in their opinions, and clerks read them. A well-timed brief with genuinely new data or a fresh legal angle can change how a court frames an issue. The classic example is the “Brandeis brief” in Muller v. Oregon in 1908, which devoted 98 of its 113 pages to factual data from factory inspectors, physicians, and social workers rather than traditional legal argument. That approach of grounding legal argument in real-world evidence became a template advocates still follow. A brief that just echoes what the parties already said is the kind courts routinely ignore.

How to File an Amicus Brief

Getting Permission

You cannot simply drop a brief on the court’s desk. In federal appellate courts, an amicus filer generally needs either written consent from all parties or leave of court through a motion.² The motion must explain the filer’s interest and why the brief would help the court.

Government filers get a pass. Under the Federal Rules of Appellate Procedure, the United States, its officers or agencies, and any state may file an amicus brief without consent and without seeking the court’s permission.² The Supreme Court follows the same principle: no motion for leave is needed when the Solicitor General files on behalf of the United States, when a federal agency’s authorized representative files, when a state attorney general files on behalf of a state, or when an authorized law officer files for a city, county, or similar entity.³ Everyone else goes through consent or motion.

Timing

Deadlines are tight. In federal appellate courts, an amicus brief must be filed no later than seven days after the principal brief of the party being supported. An amicus that supports neither party must file within seven days after the appellant’s or petitioner’s principal brief.² A court can grant permission for later filing, but missing the deadline without prior approval is a reliable way to have the brief rejected.

At the Supreme Court, the clock depends on the stage. An amicus brief supporting a petitioner at the certiorari stage must be filed within 30 days after the case is docketed or a response is called for, whichever is later. In a case set for oral argument, the brief must be filed within seven days after the brief for the party being supported.³ These deadlines cannot be extended.

Formatting and Length

The Supreme Court’s formatting rules are strict. Briefs must follow booklet format: paper sized 6⅛ by 9¼ inches, body text in a Century family font at 12-point type with at least 2-point leading, bound along the left margin with saddle stitching or perfect binding preferred.⁴ Spiral, plastic, metal, and string bindings are never allowed. Even the cover color is prescribed and signals which side the brief supports, with cream for the petition stage, light green for briefs supporting the petitioner or appellant on the merits (or supporting neither party), and dark green for briefs supporting the respondent or appellee on the merits.

Word limits vary by stage and filer. At the petition stage, an amicus brief is capped at 6,000 words. On the merits, government entities listed in Rule 37.4 get 9,000 words, while all other amicus filers are limited to 8,000.⁴ In the federal appellate courts below, an amicus brief can be no more than half the maximum length allowed for a party’s principal brief.²

Disclosure Requirements

One recurring concern with amicus briefs is that they can quietly serve as a vehicle for undisclosed lobbying. A brief that reads like neutral academic analysis might actually be drafted or funded by a party. Both the Supreme Court and the federal appellate courts require transparency to address this.

At the Supreme Court, Rule 37.6 requires that the first footnote on the first page of every amicus brief disclose whether a party’s counsel authored any part of the brief, whether counsel or a party contributed money toward preparing or submitting it, and the identity of every person or entity other than the amicus, its members, or its counsel who made a monetary contribution.³ Government filers listed in Rule 37.4 are exempt. The Court’s clerk has noted that even when no outside contributions were made, the better practice is to say so explicitly rather than stay silent.

In federal appellate courts, corporate amicus filers must include a disclosure statement similar to the one parties file, identifying parent corporations and publicly held companies that own 10 percent or more of the filer’s stock.²

Why Courts Reject Amicus Briefs

Courts are not required to accept every amicus brief submitted, and the most common reason for rejection is redundancy. The advisory committee notes to Federal Rule of Appellate Procedure 29 state that a brief bringing relevant information the court hasn’t already received is “of considerable help,” while a brief that merely repeats what the parties said “simply burdens the staff and facilities of the Court and its filing is not favored.”² The seven-day stagger between a party’s brief and the amicus deadline exists so the amicus can read the party brief and avoid repetition.

A court may also strike an amicus brief if accepting it would force a judge to recuse due to a conflict of interest. An amicus brief filed late without prior permission is almost always rejected. Beyond those grounds, courts have broad discretion, and a motion for leave that fails to articulate why the brief is useful rarely succeeds.

Can an Amicus Speak at Oral Argument?

Filing a brief is one thing. Speaking at argument is another. Under the Federal Rules of Appellate Procedure, an amicus may participate in oral argument only with the court’s permission.² Courts sometimes allow it when one of the parties is willing to share argument time. An amicus requesting independent time without extraordinary circumstances is usually turned down.

1
American Bar Association. Amicus Curiae Briefs
2
Cornell Law School / Legal Information Institute (LII). Federal Rules of Appellate Procedure Rule 29 – Brief of an Amicus Curiae
3
Cornell Law School. Supreme Court Rule 37 – Brief for an Amicus Curiae
4
Cornell Law School. Supreme Court Rule 33 – Document Preparation: Booklet Format