Ethnobiology is concerned with the social and cultural transformation of biological knowledge. Roy Ellen, who has worked among the Nuaulu people of eastern Indonesia for more than twenty years, argues here that ethnobiology is a key theoretical area of anthropological inquiry, because it relies on accessible ethnography to explain the interrelationship between collective representations and cognitive processes. He demonstrates this through a detailed analysis of Nuaulu classification of animal knowledge: the relationship between animal words and animal categories; the construction of different categories and their relationship to one another, and the actual language of classification. The classifications are shown to be context bound and socially embedded, of practical importance to their users, and to reflect an interaction between culture, cognitive processes, and the material world. This is an innovative study which takes our understanding far beyond the taxonomic abstraction characteristic of earlier work in the field.