This literature review was undertaken to explore theoretical models of depression for their potential usefulness in nursing research and practice. Depression has been accounted for by numerous theories or models of causation ; 11 theories selected from psychology, medicine and psychoanalysis and supported by empirical or experimental research were reviewed. These theories identify a variety of precipitating and predisposing factors that may affect the individual's depression. Aggression-turned-inward theory, object loss theory, ego functioning theory, personality organization theory, behavioral theory, learned helplessness theory, cognitive theory, genetic factors, and biological theories conceptualize predisposing factorrs. Only life stressors theory identifies precipitating facotrs. Each of these theories contributes to an understanding of depression, but many of them use overlapping and interrelated factors. It is also evident from recent research that there are multiple causes for depression involving an interactive effect among predisposing and precipitating factors that are both biological and psychological in origin. That is. a single theory is not useful, but perhaps a unified theory could be developed that would be helpful to nursing. This review points to the need for continuing development and testing of theories that would integrate the multiple conceptualizations of depression.