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 New Jersey, the rules for intervention in a lawsuit are governed by the New Jersey Court Rules, specifically Rule 4:33. According to this rule, a person or entity may intervene in an ongoing lawsuit if they have an interest that is related to the property or transaction that is the subject of the action. The intervenor must claim an interest that might be impaired or impeded by the disposition of the lawsuit and that interest is not adequately represented by the existing parties. The intervenor can file a motion to intervene as of right or by permission of the court. If the intervenor meets the criteria for intervention as of right, the court must allow the intervention. If the intervention is permissive, the court has discretion to allow it based on factors such as whether the intervenor's claims or defenses have a question of law or fact in common with the main action. The burden to challenge an intervention lies with the party opposing it, and they must file a motion to strike the intervention. A trial court may be found to have abused its discretion if it strikes an intervention that meets the necessary criteria, does not excessively complicate the case, and is nearly essential to protect the intervenor’s interests.