Building Transnational Networks tells the story of how a broad group of civil society organizations came together to contest free trade negotiations in the Americas. Based on research in Brazil, Chile, Mexico, the United States, and Canada, it offers a full hemispheric analysis of the creation of civil society networks as they engaged in the politics of trade. The author demonstrates that most effective transnational actors are the ones with strong domestic roots and that “southern” organizations occupy key nodes in trade networks. The fragility of activist networks stems from changes in the domestic political context as well as from characteristics of the organizations, the networks, or the actions they undertake. These findings advance and suggest new understandings of transnational collective action.