As in many areas of concern to psychology, there often has been a dichotomous perspective regarding the conceptualization of normal and deviant behavior. The dichotomy referred to is that of nature versus nurture, or biogenic versus psychogenic. Rimland (1969) suggests that the psychogenic perspective in regard to mental illness has prevailed in the past years, even though there has been little substantive evidence for this view. He further stated that in time, research would demonstrate the important role of biogenic factors in mental illness and eventually the purely psychogenic hypothesis will be abandoned. Indeed, there is now substantive evidence for biological determinants playing a major role in the genesis of most psychiatric disorders. The pendulum has moved from the extreme position of a purely psychosocial etiology to a biopsychosocial model of causation. The impetus for our theoretical and pragmatic approach to the treatment of psychiatric disorders, came from the early formulations of Flor-Henry (1976) regarding the significance of asymmetrical brain dysfunction in psychopathology. the ability to categorize psychopathology in terms of asymmetrical patterns of brain dysfunction promises a possible objective method of differential psychiatric diagnosis, independent of behavioral symptomatology. This chapter presents a theory based on 13 years of neuropsychological research utilizing a multimodal approach in the assessment (neuropsychological, brainstem, subcortical and cortical evoked potential and power spectral EEG) and treatment of psychopathology. The model proposed is multidimensional in nature, and views psychopathology as spectrum disorders with multiple causal factors interacting within a biopsychosocial matrix. The present aspect of the theory focuses on the biological determinants, as they are considered to play the major role in the genesis of psychopathology.