The study concludes by addressing a longstanding doctrinal tension in both courts and scholarship: the conflict between the rigid “triple identity” test and the intuitive belief that certain prior determinations should exert binding force, even when not strictly identical to the subsequent dispute. Portuguese procedural law, though carefully constructed, reveals gaps in daily litigation, resulting in both false negatives and false positives in the recognition of res judicata. In response, the work proposes a principled solution: binding effect should apply not to all grounds of a judgment, but specifically to preliminary issues that are prejudicial to the main claim, materially concrete, fully litigated with procedural guarantees, within the court’s competence, and materially identical to the issue later raised. Comparative insights from Anglo-American collateral estoppel show that such an extension is not foreign to civil law traditions, nor merely pragmatic, but rooted in a constitutional balance between legal certainty and procedural freedom. Ultimately, the study asserts that res judicata is not confined to the initial claim, but to the litigious object wherever it arises—including in the defense—provided it has been genuinely contested and expressly adjudicated. This reframing shifts the inquiry from “how far does res judicata go?” to “what have the parties truly litigated, and what has the judge definitively decided?”, revealing a constitutional configuration that safeguards stability without stifling legitimate contestation.