Temperament traits are linked to specific patterns in the relationships between neurotransmitters, and similar links were found between neurotransmitters and mental disorders. Temperament and mental illness are considered therefore to be variations along the same continuum of imbalance in neurochemical systems regulation of behaviour. This talk will discuss contributions of neurophysiological systems regulating consistent aspects of behaviour that emerge both as temperament traits and/or symptoms of mental disorders. Several results in clinical psychology will be discussed, with possible applications in both, clinical and forensic psychology.