This thesis is devoted to analysing the emerging relationship in the European Union between 'new governance' - epitomised by the development of the Open Method of Coordination (OMC) - and law. While some scholars have seen the project of new governance as a purely political or functional enterprise - a mechanism of 'soft law' - the thesis will argue against such a view through an empirical analysis of a particular OMC process - the OMC for social inclusion and social protection (the OMC SPSI). While on the one hand, the OMC SPSI has achieved considerable success in creating a new social policy vocabulary in Europe, the very description of the OMC as an instrument of 'soft law' has handed considerable power to frame key policy decisions to national and European executives, while depriving Parliaments and local authorities from their normal rights of scrutiny. The OMC SPSI illustrates why - far from invoking a merely 'technical' or procedural set of questions - 'new governance' is deeply implicated in debates over the future of the European welfare state. The indicators and recommendations of the method are not seen by its participants as neutral descriptors, but rather invoke competing views of the very ends of social policy in Europe. The description of new governance as soft and heterarhical does not therefore dilute its key legitimacy challenges, but makes them ever more pressing. In response, the thesis will argue for a 'constitutionalisation' of new governance. This constitutionalisation, the thesis will argue, should not be aimed at a legal 'juridification' of OMC procedures, or at re-enforcing their participatory potential, but rather at creating opportunities for political contestation and scrutiny in procedures too long the preserve of a small and mutually re-enforcing circle of executive actors. A 'republican' constitutionalisation of the OMC - one able to politicise the norms and indicators through which national social policy is being evaluated - may allow 'new governance' a last opportunity to refute accusations of executive dominance and technocratic paternalism that threaten to undermine its 'procedural' potential.