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)).
