Temperament is viewed here as referring to biologically based individual differences with main links to endocrinal-limbic regulation and to the morphology of the nervous system. Findings in psychophysiology, neuropsychology, mood disorders and personality disorders are compared to the traits described within the Functional Ensemble of Temperament (FET) model. FET consists of 12 components, which reflect the regulation of behaviour in four perspectives of functional tasks: dynamical aspects of tasks (energetic, lability and directionality), probabilistic (known task vs. uncertain), activity-specific (physical vs. verbalsocial vs. mental) and emergency aspects (involvement of emotional regulation). This review of the author's studies and the existing literature will argue for the contribution of imbalances in systems of opioid receptors, neurotransmitters, neuropeptides, and hormons, providing etiological linkages among temperament traits, mood and personality disorders.