Rule 13: Counterclaim and Crossclaim

Rule 13 governs counterclaims (claims by a defendant against the plaintiff) and crossclaims (claims by one co-party against another co-party). The most important distinction is between compulsory and permissive counterclaims.

A compulsory counterclaim is one that arises from the same transaction or occurrence as the plaintiff's claim. If the defendant has such a claim, they must assert it in the current action or lose it forever. The logic is judicial efficiency — if the claims share the same core facts, they should be resolved together. Compulsory counterclaims fall within the court's supplemental jurisdiction under 28 U.S.C. § 1367, so they do not require an independent jurisdictional basis.

A permissive counterclaim is any counterclaim that does not arise from the same transaction or occurrence. The defendant may choose to bring it in the current lawsuit or save it for a separate action. Unlike compulsory counterclaims, permissive counterclaims generally require their own independent basis for subject-matter jurisdiction.