The case discusses whether a federal court can exercise supplemental jurisdiction over additional plaintiffs whose claims do not meet the minimum amount-in-controversy requirement, but are part of the same case as plaintiffs who do meet the requirement. The Supreme Court held that if at least one named plaintiff satisfies the amount-in-controversy requirement, § 1367 authorizes supplemental jurisdiction over the claims of other plaintiffs in the same case, even if their claims are for less than the jurisdictional amount. However, in pendent-party cases involving supplemental jurisdiction over claims with additional parties, each plaintiff must satisfy the amount-in-controversy requirement separately, even in federal-question cases. Supplemental jurisdiction cannot be exercised over claims against additional defendants that fall outside the district courts' original jurisdiction. Before a federal court can conclude that supplemental jurisdiction over additional parties exists, it must satisfy itself that Article III permits it and that Congress has not expressly or implicitly negated its existence in the statutes conferring jurisdiction. The Judicial Improvements Act of 1990 enacted § 1367, which grants district courts supplemental jurisdiction over claims related to claims within their original jurisdiction, including claims that involve the joinder or intervention of additional parties. However, in civil actions where the district courts have original jurisdiction founded solely on section 1332, the district courts shall not have supplemental jurisdiction over certain claims by plaintiffs against persons made parties under Rule 14, 19, 20, or 24 of the Federal Rules of Civil Procedure, or over claims by persons proposed to be joined as plaintiffs under Rule 19 of such rules, or seeking to intervene as plaintiffs under Rule 24 of such rules, when exercising supplemental jurisdiction over such claims would be inconsistent with the jurisdictional requirements of section 1332, unless otherwise provided by Federal statute.

Justice Stevens dissents from the Court's interpretation of the jurisdictional statute, arguing that judges should consider all reliable evidence of legislative intent, including legislative history, to determine the ambiguity of a statute. He finds that the legislative history of 28 U.S.C. § 1367 supports Justice Ginsburg's interpretation of the statute. The House Report explains that § 1367 was intended to eliminate an anomaly in pre-Finley practice where a party could intervene as of right under Rule 23(a) and take advantage of supplemental jurisdiction, but not come within supplemental jurisdiction if parties already in the action sought to effect the joinder under Rule 19. The Court's decision in this case is criticized for ignoring the House Report's explicit rejection of a broad reading of the statutory text and imputing a sweeping purpose to Congress that is not supported by the Report's description of the statute.

The case concerns the interpretation of 28 U.S.C. § 1367, which allows for supplemental jurisdiction over state-law claims in certain circumstances. The Court held that Section 1367 enlarges federal diversity jurisdiction and permits access to federal court by coplaintiffs or class members who do not meet the amount-in-controversy requirement, as long as at least one coplaintiff or the named class representative has a jurisdictionally sufficient claim. The Court adopted a narrower reading of the statute, requiring original jurisdiction over a civil action before supplemental jurisdiction can attach, preserving previous cases such as Clark and Zahn and preventing the inclusion of plaintiffs or class members who do not meet the amount-in-controversy requirement. The case also discusses the significance of Mine Workers v. Gibbs, which established that federal courts can hear both federal and state-law claims when they have a common nucleus of operative fact, and Finley v. United States, which limited the scope of pendent jurisdiction established in Gibbs by requiring explicit jurisdictional authorization. The Court recognized that pendent jurisdiction and ancillary jurisdiction are two species of the same generic problem, which is under what circumstances a federal court may hear and decide a state-law claim arising between citizens of the same state. The case also discusses the recommendations made by the Federal Courts Study Committee to address issues related to the federal courts' congestion, delay, expense, and expansion. However, the lower court erred in its interpretation of Section 1367, and the Court clarified the correct interpretation.