At its core, the debate over the objective limits of res judicata is a constitutional balancing act between legal certainty—the stabilising force that ensures disputes, once resolved, remain closed—and procedural freedom—the equally vital principle that preserves space for contestation, adaptation, and the protection of individual rights. This chapter dissects that tension, arguing that the extension of res judicata to preliminary issues is not only constitutionally permissible but, under specific conditions, constitutionally required. It begins by dismantling the overly rigid “triple identity” criterion, proposing instead a doctrinal model that centers on the substantive legal relationship adjudicated, rather than its procedural form. This reconfiguration addresses the conceptual opacity of the Portuguese-inspired doctrine of the “authority of res judicata,” replacing it with a coherent, rights-based approach. For scholars and practitioners, the challenge is clear: If res judicata is a constitutional guarantee, the question is no longer whether preliminary determinations should be stabilised, but how to design legal tests and safeguards that reconcile certainty and freedom in a principled manner. The constitutional implications, as the chapter underscores, go far beyond procedural housekeeping.