Trade unions are little studied in political science apart from some pioneering works. Yet, in a developed liberal economy, unions are not just lobbies seeking for better wages, they fill a significant number of economic and social functions. The article gives a picture of unionization around the world and their situations in the major countries. Two main models emerge depending on whether the union is organized around the professions (Anglo-Saxon countries) or industries (Germany and Scandinavian countries). The differences relate to the workers concerned by the trade union action, its organization, its objectives, its repertoires of action and its relation to politics. Several factors explain the level of unionization and the difficulties that most unions face nowadays. Presence in the workplace appears to be everywhere the essential dimension. Manuscrit des auteurs. Toute citation doit se faire à partir de la version définitive parue dans l'ouvrage collectif dirigé par Yves Deloye et Jean-Michel De Waele.