Although consensus on the meaning of depression exists, theories vary widely regarding its source. Depression is essentially an affective disorder; however, because the assessment of sadness is difficult, most psychological theories of depression have focused on some nonaffective component of depression, such as activity level, cognitive processes, or interpersonal factors. The strength of these theories lies in the substantial body of therapy research which has been generated. Although new cognitive and behavioral therapies are effective with depressives, efforts to demonstrate that the source of effectiveness is in the process of the theory are usually unsuccessful. Useful theory-based research must monitor personal characteristics and stressful events in the absence of depression, distinguish stressful events which are independent of or dependent on personal characteristics, and distinguish between personal characteristics which increase the likelihood of depression from those which merely increase the likelihood of a depressive diagnosis. (NRB)