Online platforms and their ecosystems are the cornerstone of the digital economy. They have brought forth positive network effects. But they are also known for their information asymmetries, their potential for market failures and their problematic relationship with data protection law. This volume provides a detailed analysis of the current process of repositioning online platforms in the digital economy as regulators express concerns about the evolution from mere intermediaries to gatekeepers. The exclusive reliance on competition law instruments has proven to be incapable of coping with cases of platforms abusing their market power. Therefore, the book explores the European Union's new approach to digital markets consisting in the adoption or drafting of new legislative instruments, such as the Digital Markets Act, Digital Services Act, Proposal of AI Act, Proposal of Data Act, Proposal of Data Governance Act. The EU's emphasis on new regulatory ex ante instruments (as in the Digital Markets Act) calls for an assessment of their overlap or their interface with existing supranational and national competition rules. The book transcends mere competition law thinking by exploring the status of online platforms from the perspective of trade law rules, unfair competition law, data protection rules and intellectual property law. But in view of the global reach of online platforms, the risks of a jurisdiction-wise approach with conflicting regulatory strategies are all too clear. The volume therefore includes comparative studies on Australia and the USA. The potential impact of regulatory policy choices will also be assessed from the economic perspective. The book's message is not be confined to researchers and academics. It is also of great importance to practitioners in the digital sector who stand to benefit from the analysis of the law of online platforms, undertaken by a working group of renowned authors coming from different jurisdictions.