The required inclusion of a person who is not an original party to a lawsuit but whose presence in the lawsuit is necessary for it to proceed. This can be because the court would be unable to provide complete relief to the existing parties without that person, the person has an interest in the case that would be unprotected if left out, or the person would be subject to the risk of double jeopardy. A person whose inclusion in a case is mandatory is called an indispensable party. Mandatory joinder applies in both criminal and civil cases.
In federal civil cases, Federal Rules of Civil Procedure 19 governs mandatory joinder. The rule states that a person is only indispensable if their inclusion in a case would not deprive the court of subject matter jurisdiction. If a court cannot join a party because it would ruin the court’s jurisdiction or is infeasible for other reasons, the court must decide whether it can continue in “equity and good conscious” without that person or dismiss the case. FRCP 19(b) lists the factors the court should consider in making that determination. These include: the extent the absence might prejudice that person and existing parties, whether prejudice could be avoided, whether the judgment rendered in the person’s absence would be adequate, and if the plaintiff would have an adequate remedy if the case was dismissed.
In criminal procedure, mandatory joinder refers to the joining of all known claims against a defendant in a single prosecution. Some jurisdictions, such as Tennessee and West Virginia, also require that the claim arises from the same criminal transaction. Any known offenses not joined as a separate count within the same case cannot be the basis of a subsequent prosecution.
See also compulsory joinder.
[Last updated in July of 2020 by the Wex Definitions Team]