Political theorists aiming to articulate normative standards for the EU have almost entirely focused on whether or not the EU suffers from a ‘democratic deficit'. Almost nothing has been written, by contrast, on one of the central values underpinning European integration since at least the European Coal and Steel Community (ECSC), namely solidarity. What kinds of principles, policies, and ideals should an affirmation of solidarity commit us to? Put another way: what norms of socioeconomic justice ought to apply to the (...) EU? This is not an empirical or narrowly legal question. We are not trying to gauge the degree of attachment there currently is in the EU by, for example, citing the latest Eurobarometer poll. We are also not attempting to state the implicit rationale followed by the Court of Justice in its recent ‘solidarity’ jurisprudence, let alone trying to fix what the Commission might mean by it. In this article, I ask the more fundamental question underlying both the legal and the empirical questions: What principles of social solidarity ought to apply between states and citizens of the emerging European polity? This question has rarely been asked or answered by political theorists in an EU context, so we are entering largely uncharted territory. The article develops a tripartite model of EU solidarity in Section 2, and then applies it to the case of free movement of persons in Section 3. (shrink)