The biological and social aspects of ethnicity are the focus of this book, which comprises three chapters from anthropologists. Biology and society are both considered here in a balanced way, and the bidirectional nature of influence between the two factors is emphasized throughout. The history of the word 'ethnicity' and recent uses of the term in social anthropology are first discussed. The view of ethnic groups as biological entities, which pervades most academic disciplines, is contrasted to the current social anthropological orthodoxy which considers ethnic groups as self-defined social creations. The relationships between ethnicity and human biology are then explored. Social decisions and actions by ethnic groups are shown to have significant biological effects, as exemplified by genetic and epidemiological data. The last chapter presents an in-depth case study of the Basque people and shows the relevance of social and biological criteria for identifying ethnic membership.