This introductory chapter comes in two parts. It first sets out the rules of engagement: the complex set of interactions that determine the sustainability of dairy farming. The chapter starts with some basic elements/concepts involved in sustainability. It then provides a simple introduction to the principles of biochemistry and physiology that determine energy and protein exchanges in the dairy cow. This is intended as a context for other, more detailed discussions that explore particular ways to manipulate these fundamentals in the direction of net zero.