The article by Mark Armstrong explores two-sided markets, where the interaction between two groups of agents is facilitated by platforms. The key determinants of equilibrium prices in such markets are the magnitude of cross-group externalities, the nature of fees (lump-sum or per-transaction), and whether agents join a single platform or multiple platforms. The author presents three models: a monopoly platform, competing platforms, and "competitive bottlenecks" where one group joins all platforms. In a monopoly platform, prices are influenced by the cost of service, market power, and the external benefits to the other group. In competing platforms, the structure of prices is more complex, with platforms targeting one group more aggressively if it is on the more competitive side or causes larger benefits to the other group. In competitive bottlenecks, the single-homing side is treated favorably, while the multi-homing side faces higher prices, leading to a market failure where there are too few multi-homing agents. The article also discusses the impact of different tariff structures, such as uniform prices and two-part tariffs, on equilibrium outcomes.The article by Mark Armstrong explores two-sided markets, where the interaction between two groups of agents is facilitated by platforms. The key determinants of equilibrium prices in such markets are the magnitude of cross-group externalities, the nature of fees (lump-sum or per-transaction), and whether agents join a single platform or multiple platforms. The author presents three models: a monopoly platform, competing platforms, and "competitive bottlenecks" where one group joins all platforms. In a monopoly platform, prices are influenced by the cost of service, market power, and the external benefits to the other group. In competing platforms, the structure of prices is more complex, with platforms targeting one group more aggressively if it is on the more competitive side or causes larger benefits to the other group. In competitive bottlenecks, the single-homing side is treated favorably, while the multi-homing side faces higher prices, leading to a market failure where there are too few multi-homing agents. The article also discusses the impact of different tariff structures, such as uniform prices and two-part tariffs, on equilibrium outcomes.