Antitrust and Platforms: From of to in

This essay argues that the antitrust economics of platforms has moved from asking how platforms compete with one another to asking how competition is organised inside platforms. The older questions about pricing, network effects, tipping, and multi-homing remain necessary but new questions arise. Rankings, defaults, data access, and the design of the interface have become part of the competitive environment rather than the backdrop against which competition happens. They can create value by improving matching, trust, and coordination; they can also allocate attention, weaken outside options, and shift surplus. I organise recent work around this of/in distinction, emphasising the mechanisms through which platform design affects market outcomes and the corresponding implications for evidence, remedies, and institutional capacity. I close by considering AI-mediated markets, where similar questions arise at a less visible layer of choice.