This volume is the fourth in a series on depressive illness. The first volume, entitled Phenomenology of Depressive Illness, is devoted to a de- scription of depressive illness from a variety of perspectives that include that of the patient, the clinician, and the psychiatric researcher. It de- scribes the major subtypes of depressive illness and places them in the context of the life cycle. The second volume in this series is entitled Models of Depres- sive Disorders: Psychological, Biological, and Genetic Perspectives. This volume describes several major models of depressive disorders, in- cluding genetic, cognitive, interpersonal, intrapsychic, and neurobio- logical models. The third and fourth volumes deal with the biology of affective disorders in detail. These volumes are distinguished by a triaxial ap- proach. In Volume III the biology of affective disorders is described from the perspective of individual transmitter systems and neurophysio- logic and biologic processes. In Volume IV the biology of depression is addressed from the vantage point of symptom components of de- pression, and similarities and differences in the biology of depression are described compared to other psychiatric disorders with clini- cally overlapping features such as anxiety disorders or eating disor- ders. The effects on biology of comorbid conditions such as anxiety, personality disorders, alcoholism, and eating disorders are reviewed.