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 Vermont, the rules regarding intervention in a lawsuit are governed by the Vermont Rules of Civil Procedure. Specifically, Rule 24 outlines the conditions under which a non-party may intervene in an ongoing lawsuit. A person or entity may intervene as a matter of right if their interest may be practically impaired or impeded by the disposition of the case and if their interest is not adequately represented by existing parties. Alternatively, they may be allowed to intervene with the court's permission if they have a claim or defense that shares a common question of law or fact with the main action. The intervenor is not required to obtain the court's permission to file a motion to intervene, but the opposing party can challenge the intervention by filing a motion to strike. The court will consider whether the intervenor could have brought the lawsuit or parts of it, or could have defended against it, as well as whether the intervention will unduly complicate the case or is necessary to protect the intervenor's interests. A trial court may be found to have abused its discretion if it strikes an intervention that meets these criteria without a valid reason.