In this article, we examine three different cases in which health activists lobbied the World Health Organization and its member states to regulate the marketing practices of multinational companies. The campaign against tobacco manufacturers resulted in a binding treaty; the campaign against manufacturers of infant formula resulted in a non‐binding code of conduct; and the campaign against pharmaceutical manufacturers failed. We show that existing arguments regarding the success of global social movements and transnational advocacy networks fail to explain the divergent outcomes in these three cases. Instead, we argue that the effectiveness of global advocacy in these cases depended on the level of perceived legitimacy of the knowledge supporting the advocates' claims and on the prestige of the experts participating in the campaigns.