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 Montana, the rules for intervention in a lawsuit are guided by the Montana 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 the existing parties. Additionally, permissive intervention is allowed by the court's discretion for those with a claim or defense that shares a common question of law or fact with the main action. The intervenor is not required to obtain permission to intervene but must file a timely motion. If a party opposes the intervention, they bear the burden of challenging it by filing a motion to strike. The court may strike an intervention if it fails to meet the criteria for intervention, excessively complicates the case, or is not essential to protect the intervenor's interests. However, striking an intervention could be considered an abuse of discretion if the intervenor meets the criteria, the case is not unduly complicated by the intervention, and the intervention is necessary to protect the intervenor's interests.