Vasopressin/oxytocin and related peptides comprise a phylogenetically old superfamily of chemical signals in both vertebrates and invertebrates. Each peptide isoform has its own distinct receptor subtype and specific cellular action. The conservation and dispersion of vasopressin/oxytocin signalling systems across the animal kingdom attests to their functional significance in evolution. Indeed, they are involved in the physiology of fluid balance, carbohydrate metabolism, thermoregulation, immunity and reproduction. In addition, these peptides evolved a role in social behaviours related to aggression and affiliation. The focus of this chapter is the role of vasopressin/oxytocin as chemical signals in the brain altering aggressive responding in a context- and species-dependent manner. There is compelling evidence from several mammalian species including humans that vasopressin enhances aggression. The activity of the vasopressin appears linked to the serotonin system providing a mechanism for enhancing and suppressing aggressive behaviour.