Solidarity has long been considered essential to labour, but many fear that it has declined. There has been relatively little scholarly investigation of it because of both theoretical and empirical difficulties. This article argues that solidarity has not declined but has changed in form, which has an impact on what kinds of mobilization are effective. We first develop a theory of solidarity general enough to compare different forms. We then trace the evolution of solidarity through craft and industrial versions, to the emergence of collaborative solidarity from the increasingly fluid ‘friending’ relations of recent decades. Finally, we examine the question of whether these new solidarities can be mobilized into effective collective action, and suggest mechanisms, rather different from traditional union mobilizations, that have shown some power in drawing on friending relations: the development of member platforms, the use of purposive campaigns and the co-ordination of ‘swarming’ actions. In the best cases, these can create collective actions that make a virtue of diversity, openness and participative engagement, by co-ordinating groups with different foci and skills.