This chapter argues that scientific and philosophical progress in ourunderstanding of the living world requires that we abandon a metaphysicsof things in favour of one centred on processes. We identify three mainempirical motivations for adopting a process ontology in biology:metabolic turnover, life cycles, and ecological interdependence. We showhow taking a processual stance in the philosophy of biology enables usto ground existing critiques of essentialism, reductionism, andmechanicism, all of which have traditionally been associated withsubstance ontology. We illustrate the consequences of embracing anontology of processes in biology by considering some of its implicationsfor physiology, genetics, evolution, and medicine. And we attempt tolocate the subsequent chapters of the book in relation to the positionwe defend.