How do people create and maintain functional societies, despite the temptations of self and ingroup interests? What holds these societies together? Why do people give to charity or help others anonymously, even at cost to themselves? Where do moral values and judgments come from, and how do they shape behavior? Researchers working in the area of altruism, morality, and social solidarity aim to answer these and other fundamental questions about human societies. We provide an overview of this field from a range of theoretical perspectives and levels of analysis. At the micro-level, we examine theories of moral cognition, moral identities, and values. At the meso-level, we examine within-group interaction, between-group interaction, and meso-micro links, such as the influence of neighborhoods and organizations. We also consider how moral values vary at the macro-level, and socioecological theories that aim to account for this variation.