The principle of legality represents one of the most important principles of the state of law, which significantly contributes to defending the law order and the social balance. Established as a principle of the organization and functioning of the state public authorities at the Revolution of 1789 in France, the acknowledgement of the principle of legality in an act having a constitutional value marked the moment of foundation for the state based on law principles and represented a premise of creating a modern public administration. The principle is present at the level of each judiciary branch, which provides for its popularity due to its specificity. The paper aims at achieving a brief analysis of the role of the principle of legality in public law, presenting its importance in constitutional and administrative law.