The principle of ne bis in idem is a fundamental right that bars the possibility of a defendant being prosecuted more than once on the basis of the same offence, act or facts. In view of the significance of the ne bis in idem principle for the AFSJ, this chapter offers an account of the sources and elements of that principle. The ne bis in idem principle in the AFSJ has seen spectacular developments over the past two decades. The Van Esbroeck/Zolotukhin line of case law represents a prime example of fruitful substantive judicial dialectics between the two European courts. The case law on the application of the principle to the interaction between administrative, criminal and tax law raises questions, however. The problem encountered here is perhaps not so much one of ne bis in idem, but of the fundamental relationship between criminal and (punitive) administrative law. The developments in the case law suggest that we may not find a clear answer to the question of how the ne bis in idem principle should be applied in the relation between criminal, administrative and tax law until we have a sufficiently clear notion of  the precise nature of that relationship.