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 Massachusetts, the rules for intervention in a lawsuit are governed by both state procedural rules and case law. A person or entity, known as an intervenor, may seek to become a party to a lawsuit by filing a motion to intervene. This is typically done before the court has entered a judgment. The right to intervene is granted if the potential intervenor has an interest that could have led them to initiate the same lawsuit, or if they would have a defense against the lawsuit if it were brought against them. The intervenor does not need to obtain permission from the court to intervene, but the intervention can be challenged by an existing party through a motion to strike. The court may strike the intervention if it does not meet the criteria, such as if the intervention would unduly complicate the case or if the intervenor's interests are not significantly at risk. 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 for protecting the intervenor’s interests.