This article offers an overview of how Mexican unions get involved in international union alliances. It is intended to increase our understanding of the prospects for North-South union cooperation, as the ability to construct international alliances is at the heart of union responses to globalisation. Drawing on data gathered from seven Mexican unions affiliated to the International Metalworking Federation (IMF), the authors distinguish three patterns of union involvement: a localist/defensive pattern, which rests on a narrow conception of solidarity and on sporadic relations, rather limited in scope, with other unions affiliated to the IMF; a nationalist/offensive pattern characterised by a broader view of solidarity and by intense exchange of information with other IMF affiliates; and an internationalist/proactive pattern which rests on the community of interest between workers from different countries and on active cross-border coordination of action. A critical condition for cross-border alliances lies in the existence of several power resources. Without these resources a union cannot overcome the acute constraints that it faces. In conclusion, the authors discuss the prospect of union renewal and union empowerment through North-South international union alliances.