This guide covers class and aggregate litigation end to end, for lawyers building a case and for class members deciding what to do with a notice. It sets out each Rule 23 requirement with the case law that defines it, including the commonality standard after Wal-Mart v. Dukes, the predominance analysis, and the standing rule for absent members after TransUnion v. Ramirez. It explains class definition drafting, the trial plan, expert damages models, and interlocutory appeal. It covers jurisdiction and removal under the Class Action Fairness Act, the mechanics of notice, claims administration, and settlement approval under the amended Rule 23(e). It treats mass torts and multidistrict litigation as the separate track they are. It closes with tolling doctrine, fee structures, state-court variations, and a reference section.