This thesis inquires into the Legitimacy of Harmonisation of European Private law (EPL) against the background of the broader debate on the Legitimacy of the EU, and its socalled Social and Democratic deficits. While the ‘Social’ deficit has a fundamental importance for the ‘self-understanding’ of the EU and its role in private law, the ‘Democratic’ deficit impacts on the capacity of the democratic self-government in private law, which in the past proved fundamental for pulling out the market re-embedding function of private law. The legitimacy concerns increase with the expanding powers of the EU. In private law, the main triggers have been the attempts by the EU to become an exclusive locus of private lawmaking through ‘full’ or ‘maximum’ harmonisation of big parts of private law, which gave rise to concerns regarding the impact of such a competence shift on the content of private law. This thesis can be understood both as a critique of the current efforts in EPL, equally as an exploration of how to address the normative challenges of private law making in the context of a postnational polity such as the EU, which as a matter of broader concern needs to compensate for the mismatch between its factual powers and its legitimacy resources at the ‘macro’ level. The Legitimacy ‘Deficits’ of the EU are the consequence of its postnational origin – both the functional character of the EU and the mode of governance adjusted to its functional purpose. It is also for this reason that, despite all ‘democratisation’ efforts, our normative concerns regarding the legitimacy of the EU have still not been satisfied at the macro level and we need to shift our attention to legitimacy at a different scale of observation - at the micro level. The postnational entities more broadly need to seek additional legitimisation for their actions on an ad hoc basis, through the micro legitimacy of rules, decisions and procedures that lead to them. At this level, we have also a better chance to overcome the limitations of the double democratic deficit of the EU (economic rationality on the one hand, and the lack of republican dimension of its democratic institutions on the other) through designing legitimate micro procedures. Such micro legitimate procedure shall enable the co-existence of both market and non-market rationalities in the decision-making of the EU, fundamental for allowing private law to fulfil its market re-embedding function.