Accounts of solidarity typically defend it in teleological or loyalty terms, justifying it by invoking its goal of promoting justice or its expression of support for a shared community. Such solidarity seems to be a moral option rather than an obligation. In contrast, A Moral Theory of Solidarity develops a deontological theory grounded in equity. With extended reflection on the Spanish conquest of the Americas and the US Civil Rights movement, Kolers defines solidarity as political action on others' terms. Unlike mere alliances and coalitions, solidarity involves a disposition to defer to others' judgment about the best course of action. Such deference overrides individual conscience. Yet such deference is dangerous; a core challenge is then to determine when deference becomes appropriate.Kolers defends deference to those who suffer gravest inequity. Such deference constitutes equitable treatment, in three senses: it is Kantian equity, expressing each person's equal status; it is Aristotelian equity, correcting general rules for particular cases; and deference is 'being an equitable person,' sharing others' fate rather than seizing advantages that they are denied. Treating others equitably is a perfect duty; hence solidarity with victims of inequity is a perfect duty. Further, since equity is valuable in itself, irrespective of any other goal it might promote, such solidarity is intrinsically valuable, not merely instrumentally valuable. Solidarity is then not about promoting justice, but about treating people justly.A Moral Theory of Solidarity engages carefully with recent work on equity in the Kantian and Aristotelian traditions, as well as the demandingness of moral duties, collective action, and unjust benefits, and is a major contribution to a field of growing interest.