Many if not most markets with network externalities are two-sided. To succeed, platforms in  industries such as software, portals and media, payment systems and the Internet, must “get  both sides of the market on board.” Accordingly, platforms devote much attention to their  business model, that is, to how they court each side while making money overall. This paper  builds a model of platform competition with two-sided markets. It unveils the determinants  of price allocation and end-user surplus for different governance structures (pro t-maximiz-  ing platforms and not-for-pro t joint undertakings), and compares the outcomes with those  under an integrated monopolist and a Ramsey planner. (JEL: L5, L82, L86, L96)