One way of understanding Nietzsche’s thought is through the concept of force. It is this more than other ideas that allows him to range over diverse topics, ranging from cosmology to psychology and social theory, and to suggest links or analogies between them. But what is Nietzsche’s idea of force, and how is it related to that of other thinkers? Calling it a scientific concept leaves unspecified which scientific account one has in mind. Is it the Newtonian theory, or the dynamic approaches of Boscovich, Kant and Faraday, or a more modem physics? Many of Nietzsche’s uses of this concept are bound up with his reading, so that understanding them requires us to ask not just what Nietzsche meant by force, but what he believed science meant by force. In this chapter I will survey the sources of Nietzsche’s conception of force, and then look into his use of these theories to construct a model of reality corresponding to his doctrine of eternal recurrence.