All are agreed that the digital economy contributes to a dynamic evolution of markets and competition. Nonetheless, concerns are increasingly raised about the market dominance of a few key players. Because these companies hold the power to drive rivals out of business, regulators have begun to seek scope for competition enforcement in cases where companies claim that withholding data is needed to satisfy customers and cut costs. This book is the first focus on how competition law enforcement tools can be applied to refusals of dominant firms to give access data on online platforms such as search engines, social networks, and e-commerce platforms – commonly referred to as the ‘gatekeepers’ of the Internet. The question arises whether the denial of a dominant firm to grant competitors access to its data could constitute a ‘refusal to deal’ and lead to competition law liability under the so-called ‘essential facilities doctrine', according to which firms need access to shared knowledge in order to be able to compete. A possible duty to share data with rivals also brings to the forefront the interaction of competition law with data protection legislation considering that the required information may include personal data of individuals. Building on the refusal to deal concept, and using a multidisciplinary approach, the analysis covers such issues and topics as the following: – data portability; – interoperability; – data as a competitive advantage or entry barrier in digital markets; – market definition and dominance with respect to data; – disruptive versus sustaining innovation; – role of intellectual property regimes; – economic trade-off in essential facilities cases; – relationship of competition enforcement with data protection law and – data-related competition concerns in merger cases. The author draws on a wealth of relevant material, including EU and US decision-making practice, case law, and policy documents, as well as economic and empirical literature on the link between competition and innovation. The book concludes with a proposed framework for the application of the essential facilities doctrine to potential forms of abuse of dominance relating to data. In addition, it makes suggestions as to how data protection interests can be integrated into competition policy. An invaluable contribution to ongoing academic and policy discussions about how data-related competition concerns should be addressed under competition law, the analysis clearly demonstrates how existing competition tools for market definition and assessment of dominance can be applied to online platforms. It will be of immeasurable value to the many jurists, business persons, and academics concerned with this very timely subject.