Chapter 10 applies the transactions cost theory to the context of digital platforms. This chapter starts with a discussion of digital platforms and their associated ecosystems, and the sunk investment made by complementors in reliance on the ecosystem hub (the digital platform). The chapter goes on to discuss the scope for hold-up, the economic harm from the fear of holdup, and how this threat is mitigated through commercial arrangements and reputation. The chapter also discusses how consumers may suffer from the exercise of hold-up (from reduced quality or privacy), even though they are not directly charged for platform services. These ideas have already been raised (although not treated systematically) in the digital platform literature. The chapter concludes with a discussion of what the theory might imply for regulatory controls on digital platforms, including a review of the EU Digital Markets Act.