© 2015 Taylor & Francis. Labor movements have always found it difficult to reveal and transform the social relations that constitute markets. The growing transnational movements of goods, capital, and services in themselves have therefore not triggered closer trade union cooperation across borders. Transnational collective action also requires conscious choices and a mutual understanding that solidarity across borders is warranted. For this reason, this special issue of Labor History assesses the role that politicization processes play in triggering transnational union action.