What Is Res Judicata?
Once a court has issued a final judgment in a case, the same parties cannot litigate the same claims again. It prevents endless re-litigation of the same dispute.
Quick Answer
Once a court has issued a final judgment in a case, the same parties cannot litigate the same claims again. It prevents endless re-litigation of the same dispute.
Full Explanation
Res judicata is Latin for 'the matter has been decided.' It is a doctrine that bars parties from re-litigating claims that have already been fully and finally resolved by a court. Once a final judgment is entered, that judgment is conclusive — win or lose, you cannot start over.
The doctrine has two main branches. 'Claim preclusion' (the narrower meaning of res judicata) bars re-litigation of any claims that were raised or could have been raised in the first case. If you sue someone for breach of contract and lose, you cannot file a new lawsuit asserting different theories based on the same transaction. You had one chance to bring all your claims.
'Issue preclusion' (also called collateral estoppel) is narrower: it bars re-litigation of specific factual or legal issues that were actually litigated and decided in prior litigation. If a court already determined that a defendant drove through a red light, neither party can relitigate that fact in a later lawsuit arising from the same accident.
For res judicata to apply, the prior case must have involved the same parties (or those in privity with them), the same claims or issues, and must have ended in a final judgment on the merits — not a dismissal for procedural reasons like improper venue.
Real-World Example
A plaintiff sues a contractor for negligent construction of a deck and loses at trial. She cannot then file a second lawsuit claiming the same contractor breached the same contract — res judicata bars the new claim because it arises from the same underlying transaction.
Issue preclusion appeared in Parklane Hosiery Co. v. Shore (1979), where the Supreme Court allowed a private plaintiff to use a prior SEC finding of securities fraud against the company — the issue had already been decided against the defendant in another proceeding.
Why It Matters for Law Students
Res judicata is a core topic in civil procedure and is tested heavily on the bar exam. It reflects the value of finality in the legal system — courts and parties cannot endlessly relitigate disputes. Understanding the difference between claim preclusion and issue preclusion, and the requirements for each, is essential for any litigator.