It has been said in some quarters that contemporary law in Latin America has been excessively impacted by general principles; civilian lawyers (both theoretical and practical) have levied several criticisms against this widespread phenomenon, based on their own professional and academic tradition.This article shows how this new "principled" law is not so different from the historical transformations that the old Roman Law experienced.To do this, the article reviews the process through which Roman Law moved from rigorous application of texts and forms towards its interpretive and adjudicative flexibilization through principles, among which "good faith" (bona fide) played a structural role.The notion of good faith (as the structural concept that introduced flexibility in legal argument and LOS PRINCIPIOS JURÍDICOS EN EL "VIEJO"Y EN EL "NUEVO" DERECHO Examen del contraste entre el "rigor formal" y la "flexibilidad basada en principios" en la teoría jurídica romanística*