Social bonds between people and their pets are more popular than they have ever been. Yet archaeological and anthropological evidence suggests that human-pet bonds have existed throughout history, enduring despite their relative lack of practical utility or material value for humans and, in this sense, presenting a challenge to evolutionary theory. Citing abundant research, the chapter shows that the human-pet relationship should be regarded as “mutualistic,” conferring adaptive benefits on both participants For humans, animal companionship promotes social engagement and alleviates the debilitating mental and physiological effects of psychosocial stress. Animal-assisted therapeutic interventions for people with a variety of cognitive, emotional, and physical disabilities are also becoming increasingly widespread. For animals, the human-animal bond has opened a new ecological niche and allowed dramatic increases in population size. However, the chapter also raises a number of ethical concerns related to animal welfare, public health, and environmental impact.