This paper examines how law and political science have studied the role played by sovereignty claims in the EU constitutional order. Typically, it is argued, the two disciplines have studied sovereignty in the EU from parallel perspectives, with the former emphasising the dimension of the internal sovereignty of the EU and the latter focusing on member states’ enduring external sovereignty in relation to the international system. This divergence is explained by virtue of contrasting institutional units of analysis (courts vs. representative institutions). The consequence is the absence of a shared understanding of where Kompetenz-Kompetenz, the power to decide who decides in the EU system, lies. In the light of this analysis, three models of sovereignty are then discussed as models for spanning the disciplinary boundary. These are the Westphalian, post-sovereign and confederal models of sovereignty. Amongst them, the paper concludes, it is the latter that seems to offer the greatest opportunities for reconciling insights from law and political science.