Business models based on the vast collection and process of user data in nearly real-time in recent years have enabled companies to offer a wide range of innovative and customised services, often at zero prices, with substantial gains for consumers. At the same time, data-driven network effects reinforced by user feedback loops, and high economies of scale associated with information technology infrastructures, may provide companies that own the data with market power and create a tendency for markets to tip. Concern is rising that the increasing reliance and use of personal data is harmful to consumers. While some practitioners have proposed adapting competition tools and antitrust policy to tackle such issues, others believe that these can be better addressed by data and/or consumer protection agencies. This paper attempts to define Big Data and its role within a competition context, and then identifies some of the potential implications for the enforcement of competition law in the areas of merger review, abusive of dominance and cartels. It also discusses how regulations on data ownership, access and portability may affect consumer protection and competitive neutrality. It was prepared as background for an OECD discussion held in November 2016.