A panoptic overview of current governing practices evidences the proliferation of a diverse, multifaceted, and fast-evolving fauna of private regulators in several realms of public policy. Whether at the domestic, regional, or global levels, the private regulation phenomenon has been received as a conundrum, if not an overall crisis, for modern paradigms of legal and political authority. Taking the EU as a laboratory, this doctoral dissertation addresses the interpretative challenge raised by this phenomenon by elaborating an innovative account about private regulation from a legal perspective. Drawing upon examples from the fields of trade law, financial law, data protection law in ecommunications, as well as social and environmental sustainability law through supply-chains, this doctoral dissertation examines how the EU has been transforming the terms of private regulatory authority in several legal fields with a transnational reach. By integrating elements of private law with administrative law theories, in particular, I argue that these transformations may be productively conceptualized as an emergent ‘private administrative law’ that the EU has been creating, albeit still in embryonic and inchoate ways. For these purposes, the dissertation proceeds in four steps. After documenting the proliferation of private regulators in several legal fields, part I places the idea of a private administrative law as a distinctive approach to the phenomenon through an overarching mapping and review of the relevant literature about private regulation in contemporary law and legal thought. Part II elaborates upon the legal basis as well as on the jurisprudential (i.e., theoretical – normative) foundations of the idea of a private administrative law, by contrasting the legal approaches taken by the EU and the USA to private regulatory authority. In this way, I delineate the basic analytical, methodological, and normative contours that sustain the idea of a private administrative law as a new legal concept and institution. Part III marks the transition from the abstract to the concrete. By providing an in-depth examination of technical standardisation processes (in the EU’s internal dimension) and financial reporting standards (in the EU’s external dimension), I illustrate how the idea of a private administrative law distinctively contributes to understand and assess these private regulatory practices. At the same time, I discuss why this idea also clarifies many of the puzzles and contradictions currently informing legal debates in EU law and legal thought surrounding these practices. I end up this part of the dissertation giving an indication of other realms where the private administrative law framework can also be extended, laying out in this way the groundwork for a future research agenda on private authority in national or transnational governance. Finally, part IV takes a more evaluative stance on the concept by reflecting upon its capacities, challenges, risks and also its limits. With these warrants, I conclude positioning the idea of a private administrative law as a distinctive and normatively attractive conceptual framework to reflect upon the place, role, and the very significance of EU Law within a landscape of contemporary political economies characterised by an expanding topography of private regulators.