The purpose of the present work is to deeply analyze the meaning and the importance that the ne bis in idem principle (the principle of “double jeopardy”), a fundamental principle with very ancient roots, assumes with regard to each of its facets. Firstly, the essay will assess the relevance of such principle in the domestic legal framework as well as its relationship with the most importance institutes of the substantive criminal law; then, it will analyze the role covered by the ne bis in idem principle within the framework of the European Convention on Human Rights (art. 4, Protocol no. 7 of the ECHR) and the framework of the Charter of Fundamental Rights of the European Union (art. 50 of the CFREU). Focusing on the concepts of “criminal matter” (as identified by the so-called “Engel criteria”) and “idem factum” - both essential in order to define the scope of application of the ne bis in idem principle - as interpreted by the European judges, who have given to such concepts a very different content from the one adopted at a national level, the paper will show how such a different interpretation has exercised a determining influence upon the conviction of the Italian “double track-line” penalty system (“doppio binario sanzionatorio”) by the European Court of Human Rights with the famous “Grande Stevens vs. Italy” judgement. Moreover, taking a look also to the interpretation adopted by the European Court of Justice in this regard, the essay will describe the developmental stages of a long and controversial jurisprudential dialogue, having as main protagonists the judges of Strasbourg on one side and the Italian State on the other side: a dialogue concluding – for the moment – with the “A & B vs. Norway” sentence, which shocked one again the precarious balance between the Italian penalty systems and the fundamental principle of ne bis in idem. Which will be the best solution in order to overcome such impasse?