Class actions require numerous similar claims with common legal and factual questions predominating individual issues. Federal Rule 23 mandates numerosity, commonality, typicality, and adequate representation. Personal injury classes face challenges due to individual damage variations. Mass torts often proceed as multidistrict litigation instead. Common issues might include defect causation or liability standards. Individual damage trials may follow class liability findings. Notice requirements ensure due process for absent members. Settlement approval requires court finding of fairness. Opt-out rights preserve individual lawsuit options. Attorney fee structures differ from individual cases. Class certification decisions significantly impact litigation strategy.