Reciprocity is a pervasive type of social interaction in encounters, groups and organizations. Simple giving is one of the major ways of transferring goods. And others regarding social sentiments, play crucial roles in the working and in the quality of society. This volume gathers basic recent works in its main domains such as, among others, the theory of reciprocity, the public economics of transfers, the economics of the family, charities, gifts of organs, or the motivations for gift-giving. It constitutes a landmark in this rapidly expanding field of research.