Abstract : A flexible law leaves a margin of discretion to the judge. In innovating an complex environments, should the lawmaker try to specify each contigencies or should he let some “shadow areas” in the law? This dissertation contibutes to the economic analysis of legal rules characteristics literature, and it participates to the debates on the efficiency of judicial systems and on the optimal complexity of law. In the first chapter, a theoric treatment of the question of legal rules value is introduced. It describes the tradeoffs faced by a lawmaker and it justifies the existence of incomplete legal rules. The formulation of legal rules has an ex ante impact. Therefore, the issue of public authority intervention basis is raised. A flexible regime, characterized by the occurrence of harm basis for the sanction, and an act-based sanction regime are compared in the second chapter. The formulation of legal rules has also an ex post impact : the impact of the degree of precision of legal rules is analyzed in the third chapter. We show that a higher degree of precision facilitates conflict resolution. In the fourth chapter, the concept of flexibility is applied to the problem of asylum law harmonization. We show that States disparities create a demand for a flexible law.