Companion animals (or pets) form a distinctive category of domestic animals defined by their primary use as nonhuman social support providers. Companion animals have an ancient history that may precede and anticipate the original domestication of animals. Currently, more than 60% of European and American households keep pets, and their numbers are increasing rapidly in several emerging economies. The results of research over the past four decades suggest that relationships with companion animals may be beneficial to human health and well-being, though the extent of the benefits will likely depend on relationship quality. Exposure to positive relationships with pets in childhood may also predispose people to develop more empathic responses to animals later in life. In spite of these benefits, pet ownership also imposes costs, particularly in terms of environmental damage, risk to public health and threat to animal welfare. The future of these exceptional human–animal relationships will depend on striking a positive balance between the benefits and the costs.