This lecture will discuss common economic features of three industries: (i) telecommunications networks, (ii) media markets funded in part by advertising, and (iii) credit cards. In each of these industries there are two groups of participants (callers/recipients in telecommunications, readers/advertisers in media markets, and consumers/merchants with payment systems), with firms acting as competitive intermediaries between the groups. In each case, network decisions made by one participant affect the terms on which others have "access" to this participant, and leads leads to a danger of market failure.