1. Supplemental jurisdiction is the authority of United States federal courts to hear additional claims substantially related to the original claim even though the court would lack the subject-matter jurisdiction to hear the additional claims independently. 28 U.S.C. § 1367 is a codification of the Supreme Court’s rulings on ancillary jurisdiction (Owen Equipment & Erection Co. v. Kroger, 437 U.S. 365 (1978)) and pendent jurisdiction (United Mine Workers of America v. Gibbs, 383 U.S. 715 (1966)) and a superseding of the Court’s treatment of pendent party jurisdiction (Finley v. United States, 490 U.S. 545 (1989)).
2. By default, courts have supplemental jurisdiction over “all other claims that are so related . . . that they form part of the same case or controversy” (§ 1367(a)). This means a federal court hearing a federal claim can also hear substantially related state law claims, thereby encouraging efficiency by only having one trial at the federal level rather than one trial in federal court and another in state court. However, if the case is brought as a diversity action (i.e., each defendant comes from a state different than each plaintiff), there generally is no supplemental jurisdiction if such claims would destroy complete diversity. See Exxon Mobil Corp. v. Allapattah Services, Inc. Courts are also free to decline to exercise supplemental jurisdiction in specified or exceptional circumstances (§ 1367(c)).