In law, intervention is a procedure to allow a nonparty, called intervenor (also spelled intervener) 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.
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.
Intervenors 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 intervenor is currently a litigant in a case with legal issues similar or identical to the case at hand;
- if the proposed intervenor 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 intervenor is concerned that the court's decision in a particular case might be so broad as to affect the proposed intervenor'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 intervenors 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 intervenors 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 intervenor is acting altruistically. In general, the goal of the intervenor is to influence the court in making its decision, not just to "assist" the court.
Canadian courts (also courts in UK) 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 intervenor 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 intervenors.
The Attorney-General has the right to intervene in a private lawsuit if the lawsuit may affect "the prerogatives of the Crown, including its relations with foreign states". Furthermore, the Attorney-General may intervene with leave of the court where "the suit raises any question of public policy on which the executive may have a view which it may desire to bring to the notice of the court".
- Rule 24(a) 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 intervenor, 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.
- Rule 24(b) provides for 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 on 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 on 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 intervenor's claim. Supplemental jurisdiction is not permitted for intervention claims under 28 U.S.C. § 1367(b) when the original claim's federal jurisdiction was based solely on diversity and exercising supplemental jurisdiction over the intervening claim would be inconsistent with the diversity requirements of 28 U.S.C. § 1332. However, supplemental jurisdiction is permitted when the claims are so related that they form the same case or controversy.
In the courts of the State of Texas, a jurisdiction whose rules of civil procedure  differ considerably from the rules of civil procedure in federal court, a nonparty may intervene in a pending lawsuit by filing a pleading, which is typically called “plea in intervention” or “petition in intervention” without leave of the court, but any party in the pending lawsuit may object and ask for the intervention to be struck for cause. TEX. R. CIV. P. 60 ("Any party may intervene by filing a pleading, subject to being stricken out by the court for sufficient cause on the motion of any party."). While the Texas rules of civil procedure require no judicial permission and impose no intervention deadline, the common law dictates that a party may not intervene post-judgment unless the trial court first sets aside the judgment. See State v. Naylor, 466 S.W.3d 783, 788 (Tex. 2015). For the same reason, an intervenor must enter the lawsuit before final judgment to have standing to bring an appeal.
- Adams v Adams  3 All ER 572;  P 188;  3 WLR 934.
Justice, To Assist the Court: Third Party Interventions in the UK (2009)
- 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 - link is not direct
- several publications by Ian Brodie on the subject of interveners at the Supreme Court of Canada
- "To Assist The Court: Third Party Interventions in the UK" (JUSTICE, 2009)[dead link]