The book focuses on online platforms as the out-of-state regulators of social and economic issues. The analysis embraces regulatory schemes created by platforms and the interrelation between them and the state's supervision and enforcement. The volume consists of three parts: first, the foundational issues of private regulation in the entire platform economy or in its substantial parts; second, the cross-sectoral questions of platforms' private regulation, such as the enforcement of rules created by platforms under EU contract law, and the 'internal' vs. 'external' enforcement of platform regulatory frameworks; and third the judicial, doctrinal and policy discourses addressing the enforcement of the rules created by platforms in selected EU Member States. The authors seek to provide a broad outline of overlapping issues in the platforms' self-regulation and the various responses adopted in various jurisdictions.