The main goal of this dissertation is to analyze the phenomena of constitutionalization in Administrative law and the resulting reconsideration of two basic pilars underpinning this field of Law: the principles of legality and of administrative legitimacy. After some introductory considerations followed by the presentation of the main theme in the first Chapter, the author will review, in the second Chapter, the emergence and evolution of Administrative Law in order to demonstrate that, in view of the current constitutionalization of the legal system, the constitutional perspective should be added to the classic conception of Administrative Law. The third Chapter will focus on the study of the legality principle and how it evolved through times. The liberal view of this principle, the cornerstone of the Liberal Rule of Law, comes to a crisis. The Law becomes less and less the leading agent of the legal order while this role is assigned to the Constitution. The decreasing prestige of Parliament as well as the complexity and the ever-faster pace of modern life end up extending the normative power to administrative entities found outside the realm of the Legislative Branch. Therefore, institutes such as the de-legalization and the administrative reserve, as well as the institute of -autonomous regulation-, gain prominence in the current public debate. Instead of owing respect only to the formal Law, the administrative arena should also adjust itself to the legal order as a whole, which means that the -principle of jurisdicity- becomes paramount. Subsequently, in the fourth Chapter, the study will discuss the classic conceptions of legality and will demonstrate the need to reinforce the democratic legitimacy in Public Administration by means of effective instruments allowing citizens to be part of the decision-making process and of the development of administrative norms. The fifth chapter, in turn, will demonstrate how the re- consideration of the principles of legality and administrative legitimacy can be more accurately verified in the way regulatory agencies were formatted. Any discussion on the constitutionality of the regulatory model adopted by the legal order in Brazil necessarily includes compatibility with the principles under discussion The sixth chapter will conclude the study pointing out the importance of the -principle of jurisdicity- under the Democratic Rule of Law.