The intra-EU mobility of workers/persons is becoming an increasingly contentious and polarizing issue and is occupying the center stage in the so-called Brexit debate. Challenging the principle of free movement is no trivial matter. What is actually put in question are not only the foundational pillars of the single market, but of the EU as such, understood as a single (would-be) polity. The aim of this paper is to offer an analytical framing of this challenge. Section 1 provides a brief historical reconstruction of welfare state building at the national level, highlighting the salience of boundaries and of the “bounding-bonding” nexus. It also discusses the impact of European integration on the intra-EU boundary configuration in the sphere of solidarity. Section 2 illustrates the state of play as regards mobility, summarizing the findings of empirical research on the economic and financial implications of free movement and of the social security coordination regime. Section 3 argues that – in addition to economic efficiency – the principle/logic that underpins free movement is that of “hospitality”, rather than the more general principle of solidarity. The section then discusses some unintended practical implications that follow from the logic of hospitality and that lie at the basis of the increasing contentiousness around free movement. Section 4 discusses possible institutional remedies for containing political contention. The underlying assumption is that the preservation of free movement is key for the survival of the EU qua political association/community and that such preservation must be the object of an essentially political (and not merely functional) strategy on the side of EU authorities. The conclusion wraps up.