This article is based on two case studies and an empirical body of data that includes more than 100 interviews with union officials in various countries; it examines how collective union action is constructed within cross-border union alliances. Drawing on three lines of analysis, each grounded in a theory of collective action, it shows that, although the parties involved have invested considerable resources, the long-term presence of key players capable of carrying out framing duties can be a decisive factor in the development of collective action. The article also underlines how important it is for the unions involved to define a mobilizing joint project that can help develop a common sense of belonging. The findings in the two cases are uneven, and the article therefore also highlights one of the main weaknesses of cross-border alliances, namely the fact that they are dependent on stable organizational structures in the multinational corporations.