pThis book explores the implications of the U.S. Supreme Court's ruling in Ohio et al. v. American Express, and the preceding litigation, for the treatment of multisided platforms under U.S. antitrust law. It is based on a series of articles that the authors wrote either jointly or individually, leading up to and in the aftermath of the Supreme Court's decision.ppThe authors consider that the Supreme Court ruling provides valuable guidance for antitrust analysis in such markets. This book emphasizes the importance for enforcers and the judiciary to take full account of the multisided nature of certain markets, not only in payment services, but throughout the economy including other types of multisided platforms, which are particularly common in online digital markets.ppAlthough Evans and Schmalensee agree with the Supreme Court's reasoning, this book nonetheless sets out criticisms of the Supreme Court's ruling. The authors address such criticisms, based on our and other legal and econ