The origin of eating disorders as a major mental illness may be explored by investigating several related concepts. Examination of biological, social, and psychological perspectives facilitates understanding of these disorders; neurobiological origin may be especially enlightening because of its exciting relevancy to new developments in the study and understanding of mental illness in general. This focus on the neurobiology of eating disorders may better explain the relationship between the eating disorders and the mood disorders, particularly major depression. Through the understanding of this relationship, new avenues of treatment possibilities are open, consistent with interventions and pharmacological developments in the treatment of major depression. In Part I, etiological theories of anorexia nervosa and bulimia will be defined and briefly explored. For the purposes of this article, the discussion of eating disorders will focus on the classification as specified in DSM-III-R (American Psychiatric Association, 1987) specifically limited to the categories of anorexia nervosa and bulimia nervosa.