This dissertation analyzes the development and effects of transnational union networks in Brazil. Transnational union networks are formal organizational linkages created by unions sharing a common multinational employer. Networks expand core union activities across borders allowing unions from different countries to exchange information, coordinate bargaining strategies, demonstrate solidarity during labor disputes, and, in some cases, sign International Framework Agreements (IFAs) or engage in transnational social dialogue with employers. Unions have increasingly turned to transnational networking as a strategy to increase their bargaining power and combat the negative effects of neoliberal globalization. This dissertation seeks to understand the factors that promote and inhibit the development of these transnational union networks. In 2001, the Brazilian Central Única dos Trabalhadores (CUT) launched a program known as the CUT Frente às Multinacionais, or CUTMulti, to establish and develop union networks at key multinational companies operating in Brazil. By 2011, the CUTMulti had established more than fifty union networks. To answer my research question I use case studies to compare four transnational union networks participating in the CUTMulti program. My research ultimately finds that four factors influence the development of union networks. First, the participation of Global Union Federations (GUFs) and unions in the company's country of origin positively impact union network development by providing resources and institutional access to company management. Second, a series of factors including a firm's size, its industrial sector, and the overall economic climate, create the environment in which union networking occurs. Third, unions committing institutional, discursive, and financial support to networking over the long term positively affect union network development. I also find that deep inter-personal bonds of trust between union leaders participating in a union network are critical to a network's success. Finally, an employer's approach to union networking efforts, whether through acceptance or opposition, impacts the development of a union network. Employers who oppose networking present significant hurdles to union networks, while employers who accept and engage with transnational networks through IFAs or transnational social dialogue provide opportunities to expand union bargaining power.