Rapid changes are taking place in how psychologists define mental illness. Historically, mental illness has been defined based on a classification system where different conditions—for example, depression and anxiety—have been defined as discrete diagnoses. However, research on the nature of mental illness has consistently shown that mental disorder does not fall neatly into separate diagnoses but instead runs along a continuum. At the same time, major initiatives have been put forward to redefine mental illness according to neurobiological systems, which have potential to transform understanding of the causes of mental illness and its treatment. In this presentation, Dr. Ruocco will present an overview of major frameworks that redefine mental illness according to symptom dimensions and neurobiology. He will then describe his own research on the neurobiology of personality disorder and future research directions that adopt a dimensional neurobiological approach to studying mental illness.