Res judicata is the generic term for a group of related concepts concerning the conclusive effects given final judgments. Within this general doctrine there are two principal categories: (1) claim preclusion—also known as res judicata; and (2) issue preclusion—also known as collateral estoppel.

These legal doctrines generally preclude litigants from relitigating claims and issues that were previously litigated to a final resolution or judgment.

In Alabama, res judicata, or claim preclusion, prevents a party from relitigating a claim that has already been judged on its merits and received a final judgment by a competent court. This doctrine ensures that a party cannot bring an action after it has been decided upon in a previous case, thus promoting judicial efficiency and finality. Issue preclusion, also known as collateral estoppel, bars the re-litigation of factual issues that were necessarily decided in a prior lawsuit and are essential to the judgment. Under Alabama law, for res judicata to apply, there must be identity of the parties, a prior judgment on the merits, and the same cause of action in both the first and second case. For collateral estoppel to apply, the issue in question must have been actually litigated and determined by a valid and final judgment, and the determination must have been essential to the judgment. These principles are codified in Alabama case law and are consistent with federal law, which Alabama courts may look to for guidance.