in law, processes whereby additional parties or additional claims are brought into suits because addressing them is necessary or desirable for the successful adjudication of the issues.

Joinder of claims is the assertion by a party of two or more claims based on different legal premises (e.g., contract and tort). Joinder of parties is the assertion of claims for or against parties in addition to a single plaintiff and single defendant. Impleading occurs when a third party—against whom the defendant may himself have a claim—is brought into the original suit in the interests of time and efficiency. Joinder of suit occurs when two or more issues are dispensed within the same hearing. Any defendant who claims that a third party may have a portion (or more) of the liability claimed by the plaintiff has the right to bring that third party into the suit. The same right is available to a plaintiff against whom a defendant (or defendants in third-party suits) may have filed a counterclaim. The courts have tended to favour such causes of action in the interest of time and costs saved.

Joinder may be mandatory in some instances. Compulsory joinder requires the uniting of a person who must be made a party with others in a suit because his participation is necessary for a just adjudication of the matter. A plaintiff bringing several claims on a related subject must follow the procedure of joinder or risk having those several suits barred from a hearing on the ground that they constitute a multiplicity of suits. A similar procedure is available for defendants, as compulsory counterclaims also must be raised in such a case.