The principle of legality, as well as other general principles of law, is at the origin and basis of the rules. General principles take precedence over other sources of law. They are the basis of positive law. The importance of the principle of legality is undisputed in all areas, but especially in administrative law. Among other things, because the administration must adhere to a hierarchical legal system according to this principle. This is one of the principles on which administrative law is based, that is, that applicable to public administration. The main application of the principle of legality in administrative law is its power to impose sanctions. This implies that the assignment of the public administration to the sanction can only be made by law. In general, this implies that all public authorities and citizens are subject to the law and can only do what is or is not prohibited by law.
The principle of legality is linked to other general principles of law. The principle of legality is to guarantee and protect legal certainty. Although it is used throughout the legal system, it is gaining importance in the criminal, administrative and tax fields. According to the principle of legality in criminal law: the principle of legality consists in giving precedence to the law over any activity or function of public authority. Within the framework of the principle of legality, it acts in conjunction with the principle of the general interest, according to which the private interest must give way to the public or social interest, and with the principle of the separation of powers, according to which the public authority is divided into bodies with specific and limited functions for each of them. In a broader sense, the principle of reservation of rights is the set of provisions whose regulation concerns only parliaments and which must be defined in the form of a law. At the same time, it legitimizes the actions of the state by subjecting it to the same laws as citizenship. Hence the various guarantees of the judicial system in the criminal aspect. On the other hand, the administration can dictate its own rules, always within the limits established by law. Thus, from a fiscal point of view, it excludes certain sectors and matters from administrative regulation and acts as a technique protecting the rule of law. For example, the creation, modification and abolition of taxes can only be done by law.