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In law, intervention is a procedure to allow nonparties to join ongoing litigation, either as a matter of right or at the discretion of the court, without the permission of the original litigants. The basic rationale for intervention is that a judgment in a particular case may affect the rights of nonparties, who ideally should have the right to be heard.

Interveners are most common in appellate proceedings, but can also appear at other types of legal proceeding such as a trial.

In general, it is within the discretion of the court to allow or refuse an application to intervene. There are exceptions to this however (for example, under subrule 61(4) of the Rules of the Supreme Court of Canada, if the court has stated a constitutional question then the attorney general of any province or territory, or of the federal government, may intervene "as of right", i.e. without the need to be granted leave to intervene).

Courts will tend to allow an application to intervene if the applicant will provide a different perspective on the issues before the court, without expanding those issues.

Interveners are permitted in criminal matters as well as civil matters. However, courts sometimes express concern in allowing applications for intervention in criminal matters where the applicant will make arguments against the position of the accused. It sometimes is seen as unfair that the accused in a criminal matter be required to meet arguments from sources other than the prosecution.

There are several distinct reasons why someone might wish to intervene in a proceeding:

• if the proposed intervener is currently a litigant in a case with legal issues similar or identical to the case at hand;

• if the proposed intervener represents a group of people who have a direct concern in the legal issues raised in a case (for example, if the case involves deportation of a particular individual, an application for leave to intervene might be made by an interest group for the rights of refugee claimants);

• if the proposed intervener is concerned that the court's decision in a particular case might be so broad as to affect the proposed intervener's interests; in other words it would be an intervention to ensure that the court's ruling does not have "accidental" unintended effects.

It is often said that the role of interveners is to "assist" the court in making a just decision on the dispute at hand. While it is true that judges sometimes do indicate that interveners have been of aid to the court in reaching a decision, the use of the word "assist" can be seen as misleading in that it implies the intervener is acting altruistically. In general, the goal of the intervener is to influence the court in making its decision, not just to "assist" the court.

Canadian courts use the term "amicus curiae" in a more limited sense. Generally, in Canada, an amicus curiae is someone who has been specifically commissioned by the court to provide a viewpoint which the court believes is necessary and otherwise lacking. By contrast, an intervener is someone who has applied to the court to be heard on a matter. For example, the Quebec Secession Reference (a case in the Supreme Court of Canada) had one amicus curiae and several interveners.

• Rules 55 to 59 of the Supreme Court of Canada
• Rule 8 of the Civil Procedure Rules - Nova Scotia
• Intervenors Before the Supreme Court of Canada, 1997-1999: A Content Analysis (M.A. Thesis by Amanda Jane Burgess)
• book review of Friends of the Court: The Privileging of Interest Group Litigants in Canada
• several publications by Ian Brodie on the subject of interveners at the Supreme Court of Canada

In the United States federal courts, intervention is governed by Rule 24 of the Federal Rules of Civil Procedure.

Part (a) of Rule 24 governs intervention of right. A potential party (called the applicant) has the right to intervene in a case either (1) when a federal statute explicitly confers upon the applicant an unconditional right to intervene, or (2) when the applicant claims an interest relating to the property or transaction which is the subject of the lawsuit. In the second situation, in order to be admitted as an intervener, the applicant must show that its ability to protect its interest would be impeded by disposition of the case, and that its interest is not adequately represented by the current parties to the case.

Part (b) of Rule 24 governs what is called permissive intervention, which is subject to the discretion of the judge hearing the case. An applicant may be permitted by the court to intervene (1) when a federal statute confers upon the applicant a conditional right to intervene, or (2) when the applicant's claim or defense shares a common question of law or fact with the main action. Agents of the federal or state government may be permitted by the court to intervene when a party to a case relies upon a federal or state statute or executive order, or any regulation promulgated thereunder, for its claim or defense.

In both intervention of right and permissive intervention, the applicant must make a timely application to be heard. The applicant cannot sit on its rights; it must intervene as soon as it has reason to know that its interest may be adversely affected by the outcome of the pending litigation. The applicant must serve its motion to intervene upon the parties to the case, and explain its reasons for intervening in the motion papers. In addition, U.S. federal law does not allow the procedure of intervention to violate the requirements of diversity jurisdiction. The court must have either diversity jurisdiction or federal question jurisdiction over the intervener's claim, as supplemental jurisdiction is not permitted for intervention claims under 28 U.S.C. § 1367(b).

Subrin, Stephen N., Minow, Martha L., Brodin, Mark S., and Main, Thomas O. Civil Procedure: Doctrine, Practice, and Context. Aspen Publishers, 2004. ISBN 0-7355-4086-1 pp. 834-836.

• amicus curiae
• Contribution claim (legal)