This chapter focuses on the psychotherapy of individuals who suffer distress from functional somatic syndromes; specifically, idiopathic environmental intolerance (IEI). While patients believe environmental intolerances cause their distress, its origin is treated as psychological, mediated through psychophysiological systems and mechanisms associated with the stress response. Factors considered include stress and trauma premorbid to the alleged onset of IEI; somatization and its expression through affective, anxiety, and somatoform disorders; personality disorders and associated psychological defenses; motivation for the sick role; and iatrogenic suggestion and reinforcement of unsubstantiated toxicogenic theories and treatments. Psychotherapies include behavioral desensitization, cognitive-behavioral therapy, cognitive therapy, and psychotropic medications. The greatest challenge in treatment is to overcome the patient's disabling belief in a toxicogenic explanation for his or her symptoms.