Under 28 U.S.C. § 1367, a federal district court may exercise supplemental jurisdiction over state-law claims that the court would not otherwise have subject matter jurisdiction to hear, as long as the claims are part of the same case or controversy as the claims over which the court has original jurisdiction (28 U.S.C. § 1367(a)). Supplemental jurisdiction promotes judicial efficiency because all of a party's claims can be decided in one trial by the federal court, rather than in two trials by a federal and a state court. The supplemental jurisdiction statute codifies the common law doctrines of "ancillary" and "pendent" jurisdiction.

Generally, a federal court may exercise supplemental jurisdiction over claims brought by or against additional parties such as third-party defendants, parties who are required or permitted to be joined in the case and intervenors (28 U.S.C. § 1367(a)). In cases where the federal court's jurisdiction is based solely on diversity jurisdiction, however, the court does not have supplemental jurisdiction to hear claims by or against additional parties if their presence in the case would destroy complete diversity (28 U.S.C. § 1367(b)).

A district court in its discretion may decline to exercise supplemental jurisdiction over a state law claim if:

(28 U.S.C. §1367(c).)

For more information on supplemental jurisdiction, see Practice Note, Commencing a Federal Lawsuit: Initial Considerations: Subject Matter Jurisdiction.