Dunbar's Number, as the limit on the size of both social groups and personal social networks, has achieved something close to iconic status and is one of the most influential concepts to have emerged out of anthropology in the last quarter century. It is widely cited throughout the social sciences, archaeology, psychology and network science, and its reverberations have been felt as far afield as the worlds of business organization and social-networking sites, whose design it has come to underpin. Named after its originator, Robin Dunbar, whose career has spanned biological anthropology, zoology and evolutionary psychology, it stands testament to the importance of an interdisciplinary approach to human behaviour. In this collection Dunbar joins authors from a wide range of disciplines to explore Dunbar's Number's conceptual origins, as well as the evidence supporting it, and to reflect on its wider implications in archaeology, social anthropology and medicine.