Psychiatric-mental health professionals need to have a comprehensive knowledge foundation about mental illness and the theoretical underpinnings associated with it. Definitions of theory, as well as theories of mental health and illness, abound. Variation in these definitions can be influenced by or contingent on a number of factors, including the disciplinary and specialty perspective. This chapter provides an overview of various prominent theories of mental illness. Mental health and psychology are associated with numerous theories, such as grand, middle-range, and micro-level theories. The chapter describes the work of influential theorists, researchers, and practitioners from several disciplines, including but not limited to nursing, medicine, and psychology. It presents theoretical concepts and explanations of the potential etiology of mental illness from within the framework of psychodynamic, behavioral, cognitive, social, humanistic, and biological theory. The chapter includes pertinent definitions, historical background, epidemiological incidence and prevalence rates, and comparative disease burden of mental illness.