latforms that intermediate transactions between sellers and buyers have become increasingly important in the economy. People are familiar with, for example, online marketplaces (such as Amazon and eBay), payment platforms (such as Visa, MasterCard, and Paypal), and hotel booking sites (such as Booking.com and Expedia). However, there has been a great pricing puzzle associated with these platforms in that they almost universally rely on ad valorem fees, in which cases platforms charge sellers fees proportional to the transaction value plus sometimes small per-transaction fees. Given that these platforms do not incur signi…cant costs that vary with transaction value, it is puzzling why ad valorem fees are so prevalently used. In this article, we review two alternative explanations on this pricing puzzle. One theory, provided by Shy and Wang (2011) and others, emphasizes the vertical relation between the platform and the sellers. It is shown that in the case where the platform (i.e., the upstream) and the sellers (i.e., the downstream) both have market power (i.e., socalled "double marginalization") 1 , the platform extracts a higher pro…t