Private regulators operating across national borders play and increasingly important role in areas once occupied exclusively by states and state law. These new regulators and their regulatory regimes challenge state-centred conceptions of law, including public law. The Equator Principles (EP) make up a private transnational regime that seeks to regulate infrastructure project finance world-wide. It includes detailed environmental and social impact standards and procedures. After offering a primer on Thailand’s constitutional system, the chapter turns to a case study of the Equator Principles regime that explains how it works, explores its impact in Thailand, and considers its implications – practical, conceptual, and normative – for the uncertain boundaries between private law and public/constitutional law in Thailand.