Labor activists are increasingly cooperating across national borders in campaigns aimed at convincing transnational corporations to improve wages, working conditions, and labor rights. Yet to date there are no systematic studies of why some transnational labor alliances succeed while others do not. This book thus develops a causal theory of success and failure in transnational labor alliances. I hypothesize that transnational labor alliances succeed only when they exercise a type of power that threatens a corporation’s core, material interests. Moreover, workers must coordinate both within their own organizations and across national borders in order to exercise power on the international scale. Using both cross-case and within-case methods of comparative analysis, I test this hypothesis through a matched-pair analysis of six recent transnational campaigns featuring alliances spearheaded by workers from Australia, Cambodia, Indonesia, the United Kingdom, and the United States. These campaigns occurred in the shipping, retail, security services, and luxury hotel industries between 1995 and 2010. The data, which include original interviews conducted over three years of fieldwork, provide evidence that intra-union coordination, inter-union coordination, and a context-appropriate power strategy are all necessary conditions for transnational labor alliances to succeed.