This chapter views anxiety as an organized group of adaptive functions by which an organism senses, evaluates, and responds to cues of danger in its external (or internal) environment. The mechanism of evolution, natural selection, is primarily supported by evidence provided in the remarkable variation in domesticated species. One of the major difficulties that psychiatrists have with evolutionary accounts of the origin of major mental illnesses is the apparently incapacitating effects of these conditions and effects that should have led to their disappearance from the gene pool in the more hostile environments of our prehistoric past. The evolution of social relationships based on mutual attachment in mammals provides a new set of behaviors, motivational systems, and dangers within which a new variant of anxiety can evolve. Social and other environmental interactions can either intensify anxiety and/or preserve the life and comfort of the patient with anxiety disorder.