See: Estoppel, Collateral estoppel, Preclusion

In U.S. law, res judicata1 (Lat. "a thing that has been adjudged") is a doctrine that basically requires that a claim raised in one lawsuit may not involve facts that have already been raised in prior litigation, in order to avoid piecemeal litigation.

In order for res judicata to apply, the suit in question must involve (1) the "same" claim, (2) the same parties (or parties in "privity" with the parties to the prior suit), (3) a claim that has already been resolved by a valid and final judgment. Res judicata can also apply to counterclaims (claims brought by the defendant against the plaintiff) if they are "compulsory" (in which case they're also usually "transactionally related").

I. Same Claim

Four common tests are used by courts to determine whether a lawsuit presents the same claim as a previous suit. One method is to determine whether a decision in the current action would have the effect of contradicting or undoing the decision in the previous case - this test tends to be permissive toward the filing of successive lawsuits. Another, more restrictive test, is the "transactional relationship" test: does the second suit arise out of the same set of facts ("transaction") or the same cause of action as the first? If the second suit is based on the same facts or the same legal theory as the first, then it is barred. A third approach - which is not widely accepted - is to examine whether the same evidence would be presented in the second suit as in the first; of course, this approach raises the question of how much evidence has to be the same before a suit is barred. The final test, proposed by the American Law Institute (Restatement Second of Judgments § 24(1) (1982)), would likely apply res judicata to the broadest category of cases: the causes of action are the same "if they were founded upon the same transaction, arose out of the same nucleus of operative facts, and sought redress for essentially the same basic wrong." Brown v. Osier, 628 A.2d 125, 127 (Me. 1993). When there is a final judgment against a plaintiff, claims the plaintiff has against the same defendant are extinguished with regard to "all or any part of the transaction, or series of connected transactions, out of which the action arose." Beegan v. Schmidt, 451 A.2d 642, 645 (Me. 1982). This view requires all acts or occurrences that may have caused an injury to be included in the same action.

Example 1: Undoing or Contradicting the Prior Judgment

(a) A, a district attorney, initiates criminal proceedings against B, who is subsequently convicted. B sues A for malicious prosecution, alleging that A instituted a baseless prosecution in order to cause harm to B's business. In order to prove malicious prosecution2, it is necessary to show that there was no basis for prosecuting in the first place, let alone sufficient evidence to convict. Allowing B's suit to go to trial would amount to re-litigating every aspect of A's previous criminal trial, and would potentially irreconcilably contradict the A's previous criminal conviction. Thus, B's claim is precluded.

(b)Same facts as above, but B is acquitted. Here, there is no contradiction. At the very least, a judge or jury has found that there is not sufficient evidence to convict B. Thus, a claim of malicious prosecution would not contradict the result in B's criminal case.

Example 2: "Transactionally Related Facts"

(a)A and B are motorists. A runs a stop sign, plowing into B's car, causing B bodily injuries and totalling B's car. B sues A, claiming damages for the broken right leg he got in the accident, and wins. After the first suit has been concluded, B then sues A for his broken arm and ribs from the same accident. The second suit will be dismissed; it is based on the same transaction (trespass to the person) as the first. B should have sued for all physical injuries at once.

(b)Same facts as above. B sues A for all physical injuries sustained in the accident, and wins. He then proceeds to sue A for the damage to his car. The second suit may proceed. While both suits are based on the same collision, they are based on two different
transactions. The first suit was for a trespass to the person - unlawfully causing bodily injury; the second claim is for trespass to chattels - unlawfully causing damage to property.

Example 3: "Same Nucleus of Operative Facts"

A, a police officer, beats B with a billyclub while unlawfully arresting him. B sues A for false arrest and battery. After the conclusion of the false arrest and battery suit, B sues A for violation of the Fifth Amendment under § 19833. While both suits proceed on different legal theories, they are based on identical facts. Thus, they arise from the same "nucleus of operative facts." The § 1983 suit cannot be brought.

Example 4: Compulsory Counterclaims

Compulsory counterclaims are
any claim which at the time of serving the pleading the pleader has against any opposing party, if it arises out of the transaction or occurrence that is the subject matter of the opposing party’s claim and does not require for its adjudication the presence of third parties of whom the court cannot acquire jurisdiction. But the pleader need not state the claim if (1) at the time the action was commenced the claim was the subject of another pending action, or (2) the opposing party brought suit upon the claim by attachment or other process by which the court did not acquire jurisdiction to render a personal judgment on that claim, and the pleader is not stating any counterclaim under this Rule 13.

Federal Rule of Civil Procedure 13(a). Simply put, if the defendant has a claim against the plaintiff based on the same facts as the complaint filed by the plaintiff, and there are no necessary parties not subject to the court's jurisdiction, the defendant must state the claim, unless the claim is already being litigated in separate proceedings or the plaintiff's claim is based on jurisdiction over the defendant's property4.

A and B are motorists. A runs a stop sign, plowing into B's car, causing B bodily injuries and totalling B's car. B sues A, claiming damages for the broken right leg he got in the accident, and wins. After the suit has been concluded, A sues B for injuries A sustained during the accident. A's claim should have been brought as a counterclaim, because it is based on the same set of facts as B's original suit. The second suit cannot proceed.

II. Same Parties

Res judicata generally does not apply unless the parties to the second suit are the same as the parties to the first, or one or more of the parties to the second suit are in "privity5" with one or more party to the first suit. It may also apply to the successors in interest of the parties. In some cases, a person who was not a party to the original litigation may be bound by res judicata if the party played a pivotal ròle in directing the original litigation.

III. Valid and Final Judgment

Finally, res judicata will not apply unless the prior suit was resolved on the merits by a valid and final judgment. A resolution on the merits simply means that the case was resolved by determining whether the plaintiff's claim for relief was valid or whether the plaintiff's complaint stated a claim upon which relief can be granted at all, as opposed to a decision on purely procedural grounds6. The requirement that a judgment be valid means only that it was not procured by fraud, mistake, or other impropriety. A judgment will be considered final if no further steps - appeal, rehearing, etc. - can be taken in the proceedings.




1 Also known as "claim preclusion."
2 Normally, an acquittal is an element of malicious prosecution, i.e. the plaintiff has to allege and prove acquittal. This example would apply in a hypothetical jurisdiction in which it was not an element. The difference between an element and an affirmative defence such as res judicata is that the plaintiff must allege and prove the elements, while res judicata must be alleged and proven by the defendant.
3 A federal statute that allows people to sue for violations of their constitutional rights.
4 The same goes for a plaintiff who counters a defendant's counterclaim with a counterclaim.
5 Here, privity refers to a contractual or other obligation to indemnify one of the parties to the suit, e.g. the auto insurance company in one of the motorist examples.
6 E.g., Dismissal based on the statute of limitations, lack of standing to sue, lack of jurisdiction, etc. Fed. R. Civ. P. 12(b)(1)-(5),(7). A dismissal for failure to state a claim that is expressly made without prejudice (i.e. with permission to re-file) is not a final judgment on the merits.