§ 387.Generally

Intervention lets someone who is not yet a party join a case that other people already started. Subdivision (b) describes the three ways a nonparty, called an intervenor, can do it: by joining a plaintiff in claiming what the complaint seeks, by uniting with a defendant to resist the plaintiff’s claims, or by asserting something adverse to both the plaintiff and the defendant.

Subdivision (c) sets the procedure: the nonparty must petition the court for leave to intervene, by noticed motion or ex parte application, attaching a proposed complaint or answer in intervention and stating the grounds for intervening. Subdivision (d) then splits intervention into two tracks. The court must grant intervention when either a statute confers an unconditional right to intervene, or the nonparty claims an interest in the property or transaction at issue and is situated such that the case’s disposition may impair or impede that interest, unless an existing party already represents it adequately. Short of that, the court may grant permissive intervention whenever the nonparty has an interest in the matter in litigation, in the success of either party, or against both.

Subdivisions (e) and (f) cover what happens once intervention is granted: the intervenor must separately file its complaint or answer in intervention, and serve it on the other parties in the same manner as a summons — or, for a party who has already appeared, in that same manner or under the mail-service rules of Chapter 5 (commencing with § 1010) of Title 14. The existing parties then get 30 days after service to move, demur, or otherwise plead to the intervenor’s pleading, just as they would to an original complaint or answer.