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 Ohio, the rules for intervention in a lawsuit are governed by the Ohio Rules of Civil Procedure, specifically Rule 24. This rule allows a person or entity to intervene in an ongoing lawsuit if they have an interest that is not adequately represented by the existing parties. The intervenor must file a motion to intervene, which can be as of right or permissive. Intervention as of right is permitted when the intervenor claims an interest relating to the property or transaction that is the subject of the action, and they are so situated that disposing of the action may as a practical matter impair or impede their ability to protect that interest. Permissive intervention is allowed by the court's discretion when an applicant's claim or defense and the main action have a question of law or fact in common. The burden to challenge an intervention lies with the party opposing it, who must file a motion to strike. A trial court may be found to have abused its discretion if it strikes an intervention that meets the criteria of having a significant protectable interest, not complicating the case excessively, and being nearly essential for protecting the intervenor's interest.