Group size and stocking density profoundly influence livestock welfare across all farmed species. In pigs, optimum group sizes for finisher pigs are 10-20 animals; larger groups increase competition, aggression, and stress. Cattle in feedlots show increased agonistic behaviour and reduced lying time above 15m2 per animal. Broiler stocking density above 33kg/m2 is associated with elevated footpad dermatitis, leg disorders, and heat stress. Social group stability matters: repeated mixing of unfamiliar animals causes fighting injuries and chronic stress. Research demonstrates that providing sufficient resources (feeders, drinkers, space) per animal eliminates most density-related welfare problems even at moderate densities. Minimum space allowances in EU regulations are evidence-based but often represent compromise between welfare science and economic viability. Precision livestock farming sensors enable real-time density monitoring and welfare-triggered alerts.