In September 2016, the U.S. Court of Appeals for the Second Circuit issued a decision that recognized and applied important economic principles concerning the antitrust analysis of single-firm conduct in two-sided markets. The Second Circuit reversed a February 2015 decision of the U.S. District Court for the Eastern District of New York that found Amex’s “non-discriminatory provisions” (NDPs), which prohibited merchants from steering customers toward using other credit cards that charge lower merchant fees, unreasonably restrained trade and violated section 1 of the Sherman Act. Specifically, the district court found that the government had shown by a preponderance of the evidence that Amex’s use of NDPs created an environment in which credit-card networks had little incentive to lower merchant fees, which allegedly restricted interbrand competition among those networks. The Second Circuit, however, found that the district court’s analysis focused erroneously on only the merchant side of the market. Consequently, the Second Circuit reversed the district court’s conclusions that Amex possessed significant market power and that its NDPs had an actual adverse effect on competition as a whole. In a two-sided market, network externalities exist between the two sides. The value that a consumer on one side of the market derives from her consumption of the good or service increases as the number of consumers on the other side of the market increases. Hence, the proper definition of a two-sided market must focus on how a hypothetical monopolist’s small but significant and nontransitory increase in price (SSNIP) on one side of the market would affect demand on both sides of the market. Similarly, a two-sided market analysis is necessary to examine the effects that the challenged conduct has on market competition. Examining only one side of the market would necessarily distort the outcome of that analysis and could condemn legitimate business conduct that enhances, rather than decreases, consumer welfare.