new and original insights from this approach, and to clarify the conceptual aspects. The chapter first
develops a canonical model of two-sided markets for advertising, where platforms deliver content to con-
sumers and resell their “attention” to advertisers. A key distinction is drawn between free media and pay
media, where the former result from the combination of valuable consumer attention and low
ad-nuisance cost. The first part discusses various conceptual issues such as equilibrium concepts and
the nature of inefficiencies in advertising markets, and concrete issues such as congestion and
second-degree discrimination. The second part is devoted to recent contributions on issues arising when
consumers patronize multiple platforms. In this