Aggression in pigs - including mixing aggression when unfamiliar pigs are combined, resource competition, and redirected aggression (tail-biting) - is a major welfare challenge in intensive production systems. Mixing unfamiliar pigs triggers intense fighting lasting 20-60 minutes, causing skin lesions, stress, and occasionally serious injury or death. Research identifies mixing at weaning, nursery, and finisher stages as highest-risk events. Welfare-positive management strategies include: mixing at weaning (when hierarchical fighting is less severe than at older ages), providing enrichment to redirect aggression, using introduction pens to allow nose-to-nose contact before mixing, and applying repellent sprays to reduce targeted attacks. Tail-biting is a redirected oral behaviour associated with under-stimulation, inadequate nutrition, and tail length - docked tails reduce severity but do not eliminate the underlying welfare problem. Providing rooting materials (straw, compost) is highly effective in reducing tail-biting. Injury monitoring and early removal of bitten pigs prevents escalation.