The concept of the “imperativeness” of administrative acts has been of great importance for the development of the scholarship on Italian administrative law of the 20th century. The concept’s ambiguous nature enabled it to perform various different roles and to adapt to the evolution of administrative law. Its importance arises, firstly, in the context of the legal effects of administrative acts: indeed, it has been used to describe or highlight certain features of these acts that are not peculiar to administrative acts alone, but, rather, common to all unilateral acts, including those concluded within private law. Secondly, it informs the general understanding of the relations between citizens and public administrations: here, it first reflected the notion of administrations’ supremacy, but was later influenced by principles of democracy and legal protection of individual rights. Thirdly, imperativeness has been interpreted as a quality typical to all public law acts, common to administrative acts and to those issued by the other branches of government, such as statutes and judicial decisions. Finally, it has been used to distinguish between administrative and private acts, and therefore to distinguish between public law and private law and between the respective competences of ordinary and administrative courts. The present essay analyzes this variety in definition and role of the concept of imperativeness of administrative acts, while also addressing some fundamental issues of Italian administrative law and its evolution. This evolution eventually contributed to the decline of the imperativeness concept by revealing its underlying deceptiveness and the simplifications which it entailed. The concept was once important for the study of administrative law, but is now a burden that still gives rise to misunderstandings in the theory of administrative powers and acts.