Collateral Estoppel (Issue Preclusion)

Collateral estoppel, or issue preclusion, bars the relitigation of specific factual or legal issues that were actually litigated and necessarily decided in a prior proceeding. Unlike res judicata, which bars entire claims, collateral estoppel operates at the granular level of individual issues.

Four elements must be satisfied: (1) the issue in the current action must be identical to the issue decided in the prior action; (2) the issue was actually litigated — the parties presented evidence and argument on the issue; (3) the determination of the issue was necessary to the judgment — it was essential to the court's holding, not dicta; and (4) the party against whom preclusion is asserted had a full and fair opportunity to litigate the issue.

A major development in collateral estoppel is the relaxation of the mutuality requirement. Traditionally, both parties in the second action had to have been parties in the first action (mutuality). The modern trend, following Parklane Hosiery Co. v. Shore (1979), allows non-mutual offensive and defensive use of collateral estoppel.

Defensive non-mutual collateral estoppel occurs when a new defendant uses a prior judgment against the plaintiff who lost on the same issue in an earlier case. If the plaintiff had a full and fair opportunity to litigate the issue, preclusion is appropriate because the plaintiff has already had their day in court.

Offensive non-mutual collateral estoppel occurs when a new plaintiff uses a prior judgment against the defendant who lost on the same issue. Courts apply this form more cautiously because of the risk that plaintiffs will adopt a "wait and see" approach, sitting out the first case and then using a favorable result offensively.

On exams, collateral estoppel requires precise identification of the issue, verification that it was actually litigated and necessarily decided, and consideration of mutuality and fairness concerns.