Union density in France has fallen to exceptionally low levels, yet unions are able to mobilise millions of supporters against government austerity measures. Some authors therefore argue that the union revitalisation literature overemphasises density over other power resources. The article first confirms the decline of density and the scale of the challenges unions face in organising in the face of restructuring and casualisation. Second, it is argued that unions have retained some policy influence by forming strategic alliances among themselves, although pressures for fragmentation remain strong. Third, unions' mobilising capacity indicates the need to find new ways of coordinating action at all levels. The article thus not only confirms the inadequacy of density alone as a measure of union vitality but also highlights the challenges, and some opportunities, facing unions in hostile economic conditions.