Ancillary jurisdiction refers to a court’s authority to adjudicate issues and claims that the court would otherwise not have jurisdiction over but are related to a claim that is properly before the court. For example, if a plaintiff brings a lawsuit in federal court based on federal question jurisdiction, the defendant may assert a counterclaim which raises an issue of state law over which the court would not otherwise have jurisdiction.
Ancillary jurisdiction can be exercised in two situations: 1) the additional non-federal claim must be sufficiently related to the original claim, ie., the non-federal claim must be ancillary and dependant on the original claim; or 2) the ancillary claim is asserted by the defendant rather than the plaintiff. A court is more willing to excerise ancillary jurisdiction over a defendant’s ancillary claims because it was the plaintiff who chose the court in which to bring their case whereas the defendant was forced into the plaintiff’s choice.
Generally, ancillary jurisdiction can be asserted in counterclaims, cross-claims, impleader, interpleader and interventions.
The concept of ancillary jurisdiction is codified, along with the concept of pendent jurisdiction, in the supplemental-jurisdiction statute: 28 USCA § 1367.