Illness is a matter of concern in every society. Social responses to it depend both on the nature of the illness and on cultural interpretation of its significance. This study of the occurrence, recognition and explanation of illness amongst the Gnau makes use of its author's dual training in medicine and anthropology to show why, how far, and in what respects these people of a forest village in New Guinea turn to their religious and magical knowledge in the distress of illness. The analyis shows how a study of ilness can reveal belief and open an illummatlng and crucial perspective on a society's view of its world.