This article examines the rapidly developing codependency movement and appeals to psychologists to investigate this area systematically. An effort is made to clarify factors that have contributed to confusion about the meaning of codependency, and an argument is made for the use of operationally defined diagnostic criteria. Literature is reviewed on both dependency and codependency, and important differences between these two concepts are discussed. Cermak's (1986) proposition that codependency be added to the Diagnostic and statistical manual of mental disorders--revised (DSM-III-R; American Psychiatric Association, 1987) as an Axis II personality disorder also is considered. Research possibilities and clinical applications are explored in the areas of assessment and treatment and in considering the comorbidity of codependency with psychological and physical disorders.