This dissertation investigates the role of global labor in international relations. I argue that global labor is mainly comprised of two parts: national union organizations and Global Unions. Global Unions are transnational labor organizations (TLOs) with a worldwide membership that were created by national union organizations to represent their interests internationally. I contend that Global Unions perform five interrelated functions for national unions. However, due to the inherent structural weaknesses of Global Unions, it is the national unions that, in fact, remain the critical force behind global labor. Therefore, I focus on the transnational activities of national unions. I identify three conditions that result in incentives for unions to choose strategies of labor transnationalism: the shrinking of national political opportunity structures, the increasing availability of international political opportunity structures, and the adoption of a social union or social movement unionism paradigm for union revitalization. Additionally, I identify three factors that inhibit labor transnationalism among national unions: diminishing resources, turf wars, and cultural barriers. I introduce the concept of complex labor transnationalism as an alternative approach to the more limited traditional practice of labor transnationalism. I disaggregate the activities associated with complex labor transnationalism into six types: communicative transnationalism, political transnationalism, steward transnationalism, protest transnationalism, collaborative transnationalism, and steward transnationalism. Furthermore, I conduct a case study on the state of labor transnationalism in the United States concluding that while most unions take a traditional approach towards labor transnationalism there is some evidence of complex labor transnationalism. Finally, I draw several conclusions about the role of global labor in international relations and outline three areas of potential growth.