A practical guide to market definition for multi-sided platforms: how can we build on the principles in the 2024 Market Definition Notice?

Abstract The European Commission’s revised Market Definition Notice, published in 2024, extends the original 1997 Notice in a number of ways, including by explaining how markets may be defined in the presence of multi-sided platforms. This article builds on the principles in the Notice to provide a practical guide to market definition for multi-sided platforms. We consider the latest European Union and UK case law, including the Commission’s Appstore and eTraveli/Booking decisions and the UK Competition Appeal Tribunal’s judgment in BGL/Comparethemarket, and discuss the merits of the various approaches. In particular, we consider when it is appropriate to define one market or multiple ones, how the SSNIP test can be applied to incorporate network effects across the sides, and how this test can be adjusted if there is a zero price on one side, including through the use of an SSNDQ.