This chapter discusses psychosomatic medicine and somatoform disorder. Psychosomatic disorder is a term for those physical illnesses in which psychogenic as well as physiogenic factors are etiologically important. The consideration of the role of stress in the genesis of psychosomatic disorders brings us into the complicated field of etiology. In considering the role of stress in the etiology of psychosomatic disorders it is first necessary to establish a statistically significant temporal relationship between the occurrence of stress and the manifestations of the disorder and then to determine whether the stress comes under the category of essential causes or sufficient causes. Somatoform disorders as a diagnostic group have a very short history going back to 1980, when the term was introduced in the scientific literature by the American Psychiatric Association in the third edition of its Diagnostic and Statistical Manual of Mental Disorders (DSM III), though some of the disorders included in the group are known since centuries.