(a) Intervention of Right. Upon timely application anyone shall be permitted to intervene in an action: (1) when a statute of this state confers an unconditional right to intervene or (2) when the applicant claims an interest relating to the property or transaction that is the subject of the action and the applicant is so situated that the disposition of the action may as a practical matter impair or impede the applicant’s ability to protect that interest, unless the applicant’s interest is adequately represented by existing parties.
Rule 52.12.Intervention.
Part I · Rule 52: Parties · Last amended January 1, 1994 · Last verified July 22, 2026
In one sentenceRule 52.12 lets someone who is not yet a party join a pending action as a matter of right, when a statute grants it or an unrepresented interest could be impaired, or by the court's permission when the applicant's claim shares a common question with the case.
Full Text of Rule 52.12
(b) Permissive Intervention. Upon timely application anyone may be permitted to intervene in an action:
(1) when a statute of this state confers a conditional right to intervene; or (2) when an applicant’s claim or defense and the main action have a question of law or fact in common; or (3) when the validity of a statute, regulation or constitutional provision of this state, or an ordinance or regulation of a governmental subdivision thereof, affecting the public interest, is drawn in question in any action to which the state or governmental subdivision or an officer, agency or employee thereof is not a party, the court may in its discretion notify the chief legal officer of the state or governmental subdivision thereof, and the state or governmental subdivision may in the discretion of the court be permitted to intervene, upon proper application.
(c) Procedure. A person desiring to intervene shall serve a motion upon all parties affected thereby. The motion shall state the grounds therefor, and shall be accompanied by a pleading setting forth the claim or defense for which intervention is sought. The same procedure shall be followed when a statute of this state gives a right to intervene.
Amendment History
Renumbered from 52.11 and amended April 21, 1972, eff. Dec. 1, 1972. Amended Sept. 28, 1993, eff. Jan. 1, 1994.
Official Comment
Paragraph (a) is the same as Rule 24(a) of the Federal Rules of Civil Procedure except for the reference to Missouri statutes instead of statutes of the United States. Paragraphs (b) and (c) are substantially the same as prior Rule 52.11(b) and (c). Compare: Rule 24 of the Federal Rules of Civil Procedure.
Plain-English Summary
Rule 52.12(a) covers intervention of right: anyone who applies in time must be permitted to intervene when a Missouri statute confers an unconditional right to do so, or when the applicant claims an interest in the property or transaction at issue and is positioned such that the case's outcome could, as a practical matter, impair or impede the applicant's ability to protect that interest — unless the existing parties already adequately represent it. Where those conditions are met, the court has no discretion to refuse intervention.
Rule 52.12(b) covers permissive intervention, which rests in the court's discretion. It applies when a statute confers a conditional right to intervene, when the applicant's claim or defense shares a common question of law or fact with the main action, or when a case draws into question the validity of a state or local statute, regulation, or ordinance affecting the public interest, and the state or the relevant governmental subdivision is not already a party — in that last situation, the court may notify the chief legal officer of the government involved and may permit that government to intervene.
Whether the intervention is claimed as a right or sought by permission, Rule 52.12(c) requires the same procedure: the person seeking to intervene serves a motion on all parties the intervention would affect, stating the grounds for it, accompanied by a pleading that sets out the claim or defense for which intervention is sought. This procedure applies even when a Missouri statute independently gives someone a right to intervene.
Frequently Asked Questions
What's the difference between intervention of right and permissive intervention?
Intervention of right must be granted when a statute confers an unconditional right, or when an unrepresented interest in the case's subject matter could be impaired by the outcome. Permissive intervention rests in the court's discretion and applies more broadly, including whenever the applicant's claim or defense shares a common question with the case.
How does someone ask to intervene in a pending Missouri case?
Rule 52.12(c) requires the applicant to serve a motion on all affected parties, stating the grounds for intervening, along with a pleading setting out the claim or defense the applicant wants to raise.
Can the government intervene in a private lawsuit that questions a state law's validity?
Yes. When a case draws into question the validity of a state or local statute, regulation, or ordinance affecting the public interest, and the government is not already a party, the court may notify that government's chief legal officer, and the government may then seek permissive intervention.
Does having a related interest automatically entitle someone to intervene?
Not by itself. Under Rule 52.12(a), an applicant claiming an interest that could be impaired by the case's disposition is entitled to intervene only if that interest is not already adequately represented by the existing parties.
What must an intervention motion include?
A statement of the grounds for intervention, served on all parties the intervention would affect, and accompanied by a pleading setting out the claim or defense the applicant wants to assert.
Source & verification. Rule text and amendment history are
reproduced verbatim from the Missouri State & Federal Court Rules, adopted by the
Supreme Court of Missouri. Last verified July 22, 2026. ·
Official source
Also known as: intervention of rightpermissive interventionintervening in a lawsuitmotion to intervene