The foundations of volunteering, charitable giving, voluntary associations, voluntary agencies, and other aspects of the Voluntary Nonprofit Sector (VNPS) collectively and of individual voluntary action lie in various aspects of human nature and societies. These foundations may be referred to variously as altruism, morality, ethics, virtue, kindness, generosity, cooperation, social solidarity, and prosociality (eusociality). These foundations of the VNPS, and specifically of social solidarity and prosociality, are the subjects of this literature review article/book. The central goal is providing a comprehensive and interdisciplinary theoretical framework for understanding, explaining, and predicting such phenomena, based on two versions of the author’s S-Theory: (1) Individual-System-Level General S-Theory of Human Behavior, as presented briefly here and in greater detail elsewhere (Smith, 2015, 2020a, 2020b; Smith & van Puyvelde, 2016); (2) Social-System-Level General S-Theory of Collective Prosociality-Social Solidarity, as partially sketched here for the first time in print. Social-System-Level General S-Theory of collective Prosociality-Social Solidarity argues that collective social solidarity can be better explained with a broader than usual range of factors as major causal influences, beyond normative systems. Individual prosociality behavior can be best explained and understood using the author’s Individual-System-Level General S-Theory of Human Behavior. Prosociality includes (a) instrumental (task-oriented) helping behavior, such as formal and informal volunteering or charitable giving for non-household/non-immediate family persons and also informal care of residential household/immediate family persons, plus (b) expressive prosociality or sociability that involves positive interpersonal relations with one or more other persons, both in the residential household/immediate family or outside of it, based on feelings of attachment, fellowship, friendship, affection, and/or love. Prosociality and social solidarity are clearly human universals, as Brown (1991) concludes from anthropological studies on hundreds of mostly preliterate societies on all continents. Such individual human prosociality activities often have positive short- and long-term consequences for the people who do them.