The present thesis attempts, from a case study – the NIS directive – to contribute to current studies on the effectiveness of European directives and more generally on integration through the law, as well as to shed more light on the reasons why the Member States transpose directives in many different ways. The added value of this thesis lies in the analysis and comparison of the NIS Directive's transposition process across six national regulatory frameworks, those of Finland, France, Greece, Ireland, Luxembourg, and Poland in order to identify the degree of adjustment of the content of the NIS directive to national contexts. Considering that this adjustment takes place through the discretionary use of the regulatory leeway granted by the NIS Directive to the Member States of the Union, a research framework has been established with specific criteria on the basis of which the evaluation was carried out. For assessing the discretionary use of the regulatory leeway by Finland, France, Greece, Ireland, Luxembourg and Poland three hypothesis have been tested related to the degree of policy misfit, of institutional misfit and the administrative effectiveness. What emerges from this research is that the more European directives offer regulatory leeway to EU Member States, the more the national governments will use this discretionary power granted by the directives to adapt supranational policies to national contexts, affecting thus the harmonization of national legal frameworks.