Claim preclusion bars relitigation of claims that were or could have been raised in a prior action between the same parties resulting in a final judgment on the merits.

Same claim. Most courts use a transactional test — whether the claims arise from the same nucleus of operative facts — rather than asking whether the legal theories are identical. Bringing a contract claim and later a fraud claim on the same transaction is barred.

Final judgment on the merits. Includes dismissals for failure to state a claim and summary judgment. Dismissals for lack of jurisdiction, venue or joinder are not on the merits, though they preclude relitigation of those specific determinations.

Same parties or privies. Privity covers successors in interest, those whose interests were represented, and in narrow circumstances those who controlled the prior litigation.

Practical traps. Suing on some installments of a debt and reserving others; seeking only injunctive relief and later seeking damages; and litigating a declaratory action narrowly and then attempting a broader damages case.

Exceptions. Agreement of the parties to reserve claims, an express reservation in the judgment, jurisdictional inability to raise the claim in the first forum, and certain statutory schemes.

Issue preclusion is separate and narrower: an issue actually litigated, determined, and essential to the judgment.