Never has sovereignty been as fashionable as since its explanatory and normative force first came into doubt and its knell was tolled in the European Union. With the shift in authority away from the state to new sub-state, supra-state, post-state and non-state entities, an important question is whether the concept of ultimate authority or sovereignty is to be abandoned or, on the contrary, retained and, if so, in which form. This paper aims at exploring a third way that would allow us to escape from the two types of dualism that contrast state and sovereignty, first, and rejecting and saving sovereignty, second. This paper's argument is that sovereignty is neither the simple reflection of the new European and international reality nor the application of a pre-established concept whose criteria are immutable and risk corseting the post-national order. As an essentially contestable concept, sovereignty is at once a state of affairs, a question pertaining to the nature and justification of that state of affairs and a justification of the latter. The correct use of the concept of sovereignty consists therefore in constantly contesting one's conceptions of the concept and hence one's exercize of sovereignty. As such, the reflexive concept of sovereignty can be described as cooperative in the post-national constellation where sovereign entities overlap in their claims to sovereignty over the same territory and population. Read together with the principle of subsidiarity, cooperative sovereignty implies allocating competences to those authorities that are best placed to ensure the protection of shared sovereign values and principles, such as the values of democracy and fundamental rights. In the European context, cooperative sovereignty provides the normative framework for the development of a dynamic and reflexive form of constitutionalism. Through its duties of cooperation and coherence, cooperative sovereignty countervails the risks of erosion implied by constitutional pluralism, while also enhancing the legitimacy of the European polity. This can be observed in the context of difficult issues such as constitutional conflicts, legislative cooperation and, finally, multi-level constitutionalism.