The foundations of volunteering, charitable giving, voluntary associations, voluntary agencies, and other aspects of the Voluntary Nonprofit Sector (VNPS) collectively and individual voluntary action lie in various aspects of human nature and human societies. These may be referred to variously as altruism, morality, ethics, virtue, kindness, generosity, cooperation, social solidarity, and prosociality. Such foundations of the VNPS, and specifically of social solidarity and prosociality, are the subjects of this literature review article/book. The central goal here is to provide a comprehensive and interdisciplinary theoretical framework for understanding, explaining, and predicting such phenomena, based on two versions of the author’s S-Theory.