Cite as: 521 U. S. 591 (1997)
Opinion of the Court
Rule 23's requirements must be interpreted in keeping with Article III constraints, and with the Rules Enabling Act, which instructs that rules of procedure "shall not abridge, enlarge or modify any substantive right," 28 U. S. C. § 2072(b). See also Fed. Rule Civ. Proc. 82 ("rules shall not be construed to extend . . . the [subject-matter] jurisdiction of the United States district courts").15
III
To place this controversy in context, we briefly describe the characteristics of class actions for which the Federal Rules provide. Rule 23, governing federal-court class actions, stems from equity practice and gained its current shape in an innovative 1966 revision. See generally Kaplan, Continuing Work of the Civil Committee: 1966 Amendments of the Federal Rules of Civil Procedure (I), 81 Harv. L. Rev. 356, 375-400 (1967) (hereinafter Kaplan, Continuing Work). Rule 23(a) states four threshold requirements applicable to all class actions: (1) numerosity (a "class [so large] that join-der of all members is impracticable"); (2) commonality ("questions of law or fact common to the class"); (3) typicality (named parties' claims or defenses "are typical . . . of the class"); and (4) adequacy of representation (representatives "will fairly and adequately protect the interests of the class").
15 The opinion dissenting in part does not find the class-certification issues dispositive—at least not yet, and would return the case to the Third Circuit for a second look. See post, at 630-631, 641. If certification issues were genuinely in doubt, however, the jurisdictional issues would loom larger. Concerning objectors' assertions that exposure-only claimants do not satisfy the $50,000 amount-in-controversy and may have no currently ripe claim, see Metro-North Commuter R. Co. v. Buckley, ante, p. 424 (Federal Employers' Liability Act, 35 Stat. 65, as amended, 45 U. S. C. § 51 et seq., interpreted in light of common-law principles, does not permit "exposure-only" railworker to recover for negligent infliction of emotional distress or lump-sum damages for costs of medical monitoring).
613
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