This article deals with the reconfiguration of collective action by workers who live and work in different adjoining nations in Europe, i.e. cross‐border workers. Such workers take advantage of open borders, but still rely on systems of social insurance and taxation that are not quite the same in every country. The article’s central argument is that the associations representing Western European cross‐border workers — in this case study in France, Germany, Belgium, Luxembourg and Switzerland — are increasingly defending them by recourse to law, especially Community law, rather than by the militancy practised by ‘traditional’ trade unions that largely organize at the national scale. The article aims at understanding how ‘expertise’ in EC law governing transnational workers is gained and how recourse to law is developed. Cross‐border leaders focus on specialized legal activities and on lobbying the authorities of the European Community; this is their specific answer to the fact that the trade unions do not really defend cross‐border workers and that initiating action at nation‐state level is a complicated business. Finally, the article explains how a Europeanization of defence practices, which is rather original in nature and was initially ‘located’ in border areas, is being implemented and how it contributes to the ongoing debate on the multi‐scalar strategies of trade unions and social movements.