The form and function of the anxiety mechanism and its developmental history are described. Disease, like any other threat, produces an organis-mic mobilization, with physical, behavioral, and psychological components which may be vestigial or complete and which are oriented towards the organism''s survival. Anxiety is the subjective aspect of this mobilization. Danger situations typical for the successive stages of emotional development are recapitulated by the incidence of an illness or injury and produce anxiety in pure form or in the form of its psychologic derivatives. Thus the feeling of helplessness derives from the actual situation of early infancy; the feeling of threat of separation from those on whom the sick person depends stems from the dependence of the child on the mother; the reaction to the feeling of physical incapacity draws its intensity from the value placed by the developing child on its own maturing physical abilities; lastly, the feelings of loss of self esteem and of social approval derive from the child''s internalization of the cultural sanctions against weakness or unworthiness. Illness or trauma produces an emotional regression in the direction of the original states of anxiety. This may eventuate in more than necessary functional passivity and dependence; or the anxiety may be repressed and appear in the form of secondary complicating symptoms; or there may be a "flight into health" as a denial of anxiety. Given equal severity of illness or trauma, the patient''s tolerance for the attendant anxiety will vary with his constitution, with his previous success or failure in mastering anxiety-inducing threats, and with the secondary gains of illness. The signs of serious organic disease may be minimal but attendant anxiety manifestations may be none-the-less indicative of its graveness.