One recent but major policy occurrence in Justice and Home Affairs - the Treaty of Prüm (2005) - has developed within the framework of differentiated integration, thus reopening the debate over the impact of flexibility on EU integration, what causes it, and whether it should be sought by Member States at all. Whatever the consensus, the debate itself demonstrates that the very idea of differentiated integration deserves a renewed attention today ultimately because it affects, in one way or another, the performance of the EU. This article presents a critical analysis of the practice of differentiation in Justice and Home Affairs, by examining its forms, principles and effects. It discusses the literature on the subject, emphasizing the complexity of flexible integration, but reaches different conclusions. Thus, in contrast to the dominant argument, we argue that differentiation is not necessarily about deepening and/or widening EU integration. It is also, and sometimes primarily, about power and interests, two major elements that feed mistrust among Member States. In fact, we demonstrate that mistrust can cause poor differentiation. Moreover, in the absence of trust among Member States, flexibility might contribute to sub-optimal policies. Based on past research and interviews, we substantiate our claim by investigating the driving factors, rationales and consequences of the Treaty of Prüm on the institutionalization of a EU area of Freedom, Security and Justice.