Aggression is associated with inflicting damage. Anger underlies it. Links to coercion and dominance are described, as well as excitation transfer. Defensive/offensive aggression and predatory aggression are distinct. The dichotomy between reactive aggression and instrumental (proactive) aggression is described. ‘Appetitive aggression’ is conducted for intrinsic pleasure. The notion of levels of control applies to the inhibition on aggression. Explanatory models are discussed (e.g. Lorenz’s hydraulic model). Aggression can exhibit positive reinforcement. A number of brain regions are discussed (e.g. the amygdala, VTA−NAcc, hypothalamus, cortex, and periaqueductal grey). The role of hormones is reviewed. Healthy development consists in part of learning to inhibit aggression. Early exposure to toxic role models increases the chances of later showing aggression. Other things that inhibit aggression are discussed (e.g. empathy). The chapter concludes with a discussion of several situations associated with aggression: ostracism, xenophobia, dehumanization, the weapons effect, media violence, alcohol, and traumatic brain injury.