To understand the motivation for fighting it is necessary to provide some analysis of the physiological and biological make-up of human beings as a species. This chapter will utilise up-to-date research on interpersonal violence across different disciplines including anthropology, biology, cognitive evolutionary psychology, neuroscience, physiology, anatomy, and palaeontology. The focus is on the biological underpinnings of violent action and especially how human violence differs from the aggressive behaviour of other species. This chapter scrutinises and critiques the dominant essentialist interpretations which attempt to explain human behaviour in terms of biological or psychological givens. I argue that the recent experimental studies across different disciplines indicate that interpersonal violence is complex and shaped by changing structural contexts. Unlike most carnivorous mammals, human beings lack bodily entailments for aggressive behaviour and hence their increased capacity for violence has distinctly non-biological origins. To compensate for their individual physical incompetence in belligerence, human beings had to devise effective social and organisational mechanisms for violent action. Hence biology plays some role in the human capacity for aggressive behaviour, but it largely does not determine or even shape much of human violent action. Psychology is also relevant in this context but neither biology nor psychology can adequately explain the enormous contextual and historical variation that characterises human relationships with close-range violence. Instead, violent action entails the interlocking presence of organisational capacity, ideological penetration, and micro-interactional social tuning.