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 Pennsylvania, the rules for intervention in a lawsuit are governed by the Pennsylvania Rules of Civil Procedure. Under these rules, a non-party to a lawsuit may intervene by filing a petition for intervention, typically before the final judgment is entered. The right to intervene is granted if the potential intervenor has a direct interest in the outcome of the case that is not adequately represented by the existing parties. This could be because the intervenor could have initiated the same lawsuit or has a defense that would defeat the claim. The court has the discretion to allow or deny the intervention, but it may be considered an abuse of discretion if the intervenor meets the criteria of having a direct interest, the intervention does not unduly complicate the case, and the intervention is necessary to protect the intervenor's interests. If a party opposes the intervention, they bear the burden of challenging it by filing a motion to strike the intervention. It is important for anyone considering intervention to consult with an attorney to understand the specific requirements and timing for filing a petition to intervene in Pennsylvania.