Abstract
This article examines the tension between the contemporary expansion of constitutional adjudication and the separation of powers. Its central question is how far a constitutional judge may go when interpreting the Constitution, reviewing legislation, protecting fundamental rights, and applying conventionality control without replacing the legislature, the executive, or the constituent power. Using a qualitative legal methodology that combines doctrinal, historical, comparative, hermeneutic, and case-law approaches, the study reconstructs the transition from the legislative State to the constitutional State and contrasts diffuse, concentrated, and Latin American models of constitutional review. As its main result, it proposes a functional theory of judicial limits based on eight criteria: a genuine constitutional issue, a verifiable constitutional contradiction, a semantically plausible consistent interpretation, an omission linked to an express constitutional mandate, exceptional and time-bound modulation, conventionality control within jurisdiction, rights protection supported by objective reasoning, and respect for constitutional identity. The article concludes that constitutional legitimacy does not stem from unlimited judicial creativity, but from the combination of independence, enhanced reasoning, proportionality, institutional deference, and judicial self-restraint.