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 York, the rules for intervention in a lawsuit are governed by the New York Civil Practice Law and Rules (CPLR). According to CPLR Article 10, a person or entity may intervene in a lawsuit if they have a claim or defense that shares a common question of law or fact with the main action. The potential intervenor must file a motion to intervene, which the court will consider based on factors such as whether the intervention will unduly delay the resolution of the case or prejudice the rights of the original parties. The intervenor must demonstrate an interest in the outcome of the case, and the court has discretion to allow or deny the intervention. If the intervenor could have initiated the same lawsuit or has a defense that could defeat the lawsuit, they may have a right to intervene. However, the court will strike the intervention if it finds that it would overly complicate the case or is not necessary to protect the intervenor's interests. The party opposing the intervention bears the burden of challenging it. A trial court's decision to strike an intervention can be considered an abuse of discretion if the intervenor meets the criteria for intervention, the case will not be unduly complicated, and the intervention is essential to protect the intervenor's interests.