The mental health care system has historically marginalized individuals with intellectual and developmental disabilities (I/DD). Until the mid-1980s, many clinicians doubted that individuals with I/DD were capable of depression (Sovner & Pary, 1993). Although it is now generally accepted that individuals with I/DD do have depression, they may not be treated or may be inappropriately treated (Matson et al., 2000). A historicist perspective takes into account the effect of marginalization on science and practice. Depression has both biological and psychosocial aspects. Key groups of theories regarding the psychosocial aspects of depression include psychodynamic/psychoanalytic, behavioral, cognitive, and ecological/interpersonal theories (Clark, Beck, & Alford, 1999; Joiner, Coyne, & Blalock, 1999). The application of psychosocial theories of depression to individuals with I/DD continues to reflect their marginalization and oppression. Behavioral theories of depression are limited in their conceptions for research, identification, and treatment of depression but continue to be used widely with individuals with I/DD. Cognitive theories of depression are widely used in research and treatment of depression in the general population, but have limited usage among individuals with I/DD. Interpersonal theories of depression are used in the general population and have many benefits, but are only now being investigated for use with individuals with I/DD. In this article, theories of depression as applied to individuals with I/DD are discussed from a historicist perspective.