This chapter makes no attempt to present a definitive exposé of the relationship between Schengen free movement rights and the principle of ne bis in idem. Although elements of the basic structure of that relationship are relatively fixed, the questions that it raises are complex and sensitive. In the absence, thus far, of legislative action by the Council, it has fallen to the European Court of Justice (ECJ) to define a number of core concepts in the contexts of references for preliminary rulings that have been made to it by national courts. In so doing, it has necessarily found itself choosing between two desirable goals that are partially irreconcilable with each other: liberalised free movement of persons within the ‘Schengen Area’, on the one hand, and the Union’s objective of ‘provid[ing] citizens a high level of safety within an area of freedom, security and justice’, on the other hand.