Res Judicata (Claim Preclusion)

Res judicata, or claim preclusion, is one of the two preclusion doctrines (along with collateral estoppel/issue preclusion) that promote finality in litigation. It prevents a party from relitigating a claim that was or could have been raised in a prior action that resulted in a final judgment on the merits between the same parties or those in privity with them.

Three elements must be satisfied: (1) a final judgment on the merits in the prior action; (2) identity of the parties (or those in privity); and (3) the same claim or cause of action. The most critical and often most contested element is the scope of the "claim" — how broadly to define what claims are precluded.

The majority of jurisdictions follow the transactional test from the Restatement (Second) of Judgments: all claims arising out of the same transaction or occurrence as the original claim are barred, whether or not they were actually raised. This broad approach ensures that parties cannot split a single dispute into multiple lawsuits. If a plaintiff sues for personal injury from an accident and later tries to sue for property damage from the same accident, res judicata bars the second suit.

A "final judgment on the merits" includes default judgments, summary judgment, and trial verdicts. Voluntary dismissals with prejudice also qualify. Dismissals for lack of jurisdiction, improper venue, or failure to join a party are generally not on the merits and do not trigger claim preclusion.

Res judicata also bars the parties from raising claims they "could have" raised but did not — even if the claim involves a different legal theory. This is the most powerful aspect of the doctrine: it requires parties to litigate all related claims in a single action.

On exams, res judicata appears when a party tries to bring a second lawsuit after losing the first. The key analysis is whether the second suit involves the same transaction or occurrence as the first.