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 Washington State, the rules regarding intervention in a lawsuit are governed by the Washington State Rules of Civil Procedure (CR). Specifically, CR 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 could be impaired by the outcome of the case, and if their interest is not adequately represented by the existing parties. The intervenor is not required to obtain permission from the court to intervene, but must file a timely motion to intervene. If a party to the lawsuit opposes the intervention, they must file a motion to strike the intervention. The court will allow the intervention if the intervenor could have initiated the same lawsuit, or if they could defend against it if the action were brought against them. The court may strike an intervention if it finds that the intervention would unduly complicate the case or if the intervenor's interests are not significantly at risk. However, if the intervenor meets the criteria for intervention, does not excessively complicate the case, and the intervention is necessary to protect their interests, then striking the intervention could be considered an abuse of discretion by the trial court.