Solidarity is a widespread but under-explored phenomenon. In this paper, I give a philosophical account of solidarity, answering three salient questions: What motivates acts of solidarity? What unifies different acts into tokens of a single type of act, one of solidarity? And what values do acts of solidarity exhibit? The answer to all three, I argue, involves a certain way of relating to others: identifying with them on the basis of shared features, and identifying with the larger group that one and the others both belong to.