A person or entity who is not a party to a lawsuit generally may intervene in the lawsuit and become a party to it by filing a plea in intervention prior to the entry of the court’s judgment—provided the court does not reject (strike) the intervention. An intervenor is not required to secure the court’s permission to intervene, and the party who opposed the intervention has the burden to challenge it by filing a motion to strike.

A person or entity has a right to intervene if the intervenor could have brought the same lawsuit, or any part of it in his own name—or, if the action had been brought against him, he would be able to defeat the lawsuit, or some part of it.

A trial court abuses its discretion by striking an intervention if (1) the intervenor meets the above test, (2) the intervention will not complicate the case by an excessive multiplication of the issues, and (3) the intervention is almost essential to effectively protect the intervenor’s interest.

In California, the right to intervene in a lawsuit is governed by the California Code of Civil Procedure. A person or entity not originally a party to a lawsuit may intervene by filing a petition or pleading with the court before the court enters judgment. The intervenor does not need to obtain permission from the court to intervene, but the party opposing the intervention can challenge it by filing a motion to strike the intervention. The criteria for intervention include the potential intervenor's ability to have brought the same lawsuit or a part of it, or to defend against it if the action had been brought against them. The court may be found to have abused its discretion if it strikes an intervention when the intervenor meets the criteria, the intervention does not excessively complicate the case, and the intervention is nearly essential to protect the intervenor’s interests. These provisions ensure that parties with a significant interest in the outcome of a case have the opportunity to be heard.