Compassion for others and social support have survival value and health benefits. Although compassion is sometimes considered uniquely human, critical components of compassion have been described in nonhuman mammals. Studies originally conducted in social mammals and now in humans have implicated neuropeptide hormones, especially oxytocin, in social cognition, a sense of safety, and the capacity of sociality to permit compassionate responses. In contrast, the related peptide vasopressin and its receptor may be necessary for forming selective relationships and for the apparently paradoxical effects of oxytocin, which can include increases in fear and avoidance. Oxytocin and vasopressin may contribute to sex differences in compassion. Furthermore, among the processes through which oxytocin and vasopressin influence behavior and health are complex effects on the autonomic nervous system. Knowledge of the mechanisms underlying the benefits of compassion offers new insights into the healing power of positive social behaviors and social support.