Res judicata (from "res iudicata", Latin for "a thing decided"), more commonly res judicata in legal usage, is a common law doctrine meant to bar relitigation of cases between the same parties in court.

Once a final judgment has been handed down in a lawsuit, subsequent judges who are confronted with a suit that is identical to or substantially the same as the earlier one will apply res judicata to preserve the effect of the first judgment. This is to prevent injustice to the parties of a case supposedly finished, but perhaps mostly to avoid unnecessary waste of resources in the court system. Res judicata does not merely prevent future judgments from contradicting earlier ones, but also prevents them from multiplying judgments, so a prevailing plaintiff could not recover damages from the defendant twice for the same injury.

Scope

Res judicata includes two related concepts: claim preclusion, and issue preclusion (also called collateral estoppel), though sometimes res judicata is used more narrowly to mean only claim preclusion.

Claim preclusion focuses on barring a suit from being brought again on a legal cause of action that has already been finally decided between the parties.

Issue preclusion bars the relitigation of factual issues that have already been necessarily determined by a judge or jury as part of an earlier claim.

It is often difficult to determine which, if either, of these apply to later lawsuits that are seemingly related, because many causes of action can apply to the same factual situation and vice versa. The scope of an earlier judgment is probably the most difficult question that judges must resolve in applying res judicata. Sometimes merely part of a subsequent lawsuit will be affected, such as a single claim being struck from a complaint, or a single factual issue being removed from reconsideration in the new trial.

Exceptions to application

Res judicata does not restrict the appeals process, which is considered a linear extension of the same lawsuit as it travels up (and back down) the appellate court ladder. Appeals are considered the appropriate manner by which to challenge a judgment rather than trying to start a new trial, and once the appeals process is exhausted or waived, res judicata will apply even to a judgment that is contrary to law.

However, there are limited exceptions to res judicata that allow a party to attack the validity of the original judgment, even outside of appeals. These exceptions—usually called collateral attacks—are typically based on procedural or jurisdictional issues, based not on the wisdom of the earlier court's decision but its authority or competence to issue it. A collateral attack is more likely to be available (and to succeed) in judicial systems with multiple jurisdictions, such as under federal governments, or when a domestic court is asked to enforce or recognize the judgment of a foreign court.

Failure to apply

When a subsequent court fails to apply res judicata and renders a contradictory verdict on the same claim or issue, if a third court is faced with the same case, it will likely apply a "last in time" rule, giving effect only to the later judgment, even though the result came out differently the second time. This situation is not unheard of, as it is typically the responsibility of the parties to the suit to bring the earlier case to the judge's attention, and the judge must decide how broadly to apply it, or whether to recognize it in the first place.

Application in Civil Law (Legal System)

The civil law doctrine of res judicata is much narrower in scope than its common law counterpart.

In order for a second trial to be dismissed on a motion of res judicata in a civilian jurisdiction, the trial must be identical to the first trial in the following manner: (1) identical parties, (2) identical theories of recovery, and (3) identical demands in both trials. In other words, the issue preclusion or collateral estoppel found in the common law doctrine of res judicata is not present in the civilian doctrine. In addition if all else is equal between the two cases, minus the relief sought, there will be no dismissal based on res judicata in a civil law jurisdiction.

While most civilian jurisdictions have slightly broadened the doctrine thru multiple exceptions to these three requirements, there is no consensus on which exceptions ought to be allowed.

Note: Louisiana (USA), a civil law jurisdiction, has in the last twenty years, begun to follow the common law doctrine of res judicata.