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 Arizona, the rules regarding intervention in a lawsuit are governed by the Arizona 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 in a lawsuit if they have an interest that is related 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. The intervenor must claim an interest that might be impeded by the disposition of the case and that the existing parties may not adequately represent their interest. The intervenor is not required to obtain permission to intervene but must file a timely motion to intervene before the court's judgment. If the intervenor meets the criteria for intervention and their involvement will not unduly complicate or delay the proceedings, the court is generally expected to allow the intervention. If a party opposes the intervention, they bear the burden of challenging it by filing a motion to strike. A court may be found to have abused its discretion if it strikes an intervention that meets the legal requirements, does not excessively complicate the case, and is nearly essential to protect the intervenor's interests.