This paper examines the role of European Union law as a benchmark for the legitimacy of administrative acts within national legal systems, with particular reference to the Italian experience. It analyses the complex relationship between domestic legal norms and eu law, highlighting the constitutional basis of the primacy of eu legislation and reviewing key judicial interpretations at both European and national level. The discussion categorizes the defects of administrative acts – whether voidable or void – when they violate eu standards, with particular focus on the notion of invalidity on grounds of incompatibility with eu law and its procedural and substantive consequences. Drawing on a wide range of case law, including landmark rulings on the interplay between eu directives and national administrative measures, the paper illustrates how breaches of eu law may lead to annulment and, in the most serious cases, to nullity of administrative acts, as well as to their direct disapplication by national courts. The study ultimately calls for a more coherent harmonization between national and eu norms, in order to ensure the effective protection of public interests and to safeguard the principles of legal certainty and equal treatment within the European legal space.