Solidarity is widely held to be an under-theorized, elusive, or vague notion, and there is no clear-cut canon of theories of solidarity, but there are some core intuitions on this subject that rival theories try to capture in different ways. One such core intuition is that solidarity concerns people who share their lives and whose fates are tied together—social solidarity, civic solidarity, or group solidarity are related to the strength of ties of dependency and mutual support of people who are “in the same boat.” Another core intuition is that solidarity can be extended even beyond one’s own society, community, or group—maximally to the whole of humankind. Nonexclusive human solidarity can play a vital role in sustaining moral standards and for example in the collective measures against climate change or a pandemic. A third core intuition is that solidarity can be needed and expressed in struggles against injustice or wrongs of various sorts. If the first core idea of solidarity concerns the normal stages of society, the third concerns the even revolutionary struggles to change important aspects of the existing forms of life. The metaphor of “being in the same boat” may seem suspect and misleading when attention is paid to the injustices of current arrangements—instead, what is needed is political solidarity in the attempt to fight those injustices. A fourth core intuition is that the dark side of solidarity raises suspicion: An internally solidary group may be repressive of the individuality of the members, it may be parochial and sometimes even lead to a dehumanization of outsiders, and it may be exercised in pursuit of unjustifiable ends. These forms of solidarity are discussed in the introduction (“Solidarity: Toward More Detailed Conceptions”). Among the theoretical questions concerning solidarity are, first of all, what exactly is it? Is it a specific type of relationship one can have (like friendship), or can any relationship, group, or way of acting be more or less solidary (like being friendly toward anyone, not just one’s friends)? Is solidarity a certain kind of action or a motivational basis out of which one can act? What sorts of things can be solidary (acts, attitudes, relationships, groups, practices, etc.), and can solidarity be realized or expressed via coercively sanctioned institutions? When macro phenomena are explained by microfoundations, is solidarity something to be explained or something that explains? Is solidarity a descriptive or evaluative notion, or both? Can solidarity be something bad? (“The Nature of Solidarity”). Normative questions concerning solidarity include: What kind of reasons or duties are there for being solidary? What is their relation to universalistic modern morality? What is human solidarity? (“Moral Solidarity”). What does thicker societal or in-group solidarity add to the universal demands of human solidarity? What is the relationship of solidarity to justice, democracy, social freedom or welfare state institutions? (“Perspectives on Societal Solidarity”). What is solidarity in the context of political struggles and social movements for change? (“Political Solidarity”). In what sense can these forms of solidarity be global? (“Solidarities in Global Contexts”).