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.

Intervenors 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 that, however. For example, under subrule 61(4) of the Rules of the Supreme Court of Canada, if the court has stated a constitutional question, the attorney general of any province or territory or of the federal government, may intervene "as of right," 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 if the applicant will make arguments against the position of the accused. It sometimes is seen as unfair for the accused in a criminal matter to be required to meet arguments from sources other than the prosecution.

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

It is often said that the role of intervenors is to "assist" the court in making a just decision on the dispute at hand. It is true that judges sometimes indicate that intervenors have aided 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 and British 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. In 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.