The taxonomy of the most consistent, biologically based individual differences continues to be a topic of hot debates in behavioral sciences for more than a century. Applications of such taxonomy extend from the structure of classification of mental disorders to a structured assessment within various fields of psychology and psychiatry. This chapter reviews convergence between several distant disciplines: neurochemistry, kinesiology, clinical psychophysiology and differential psychology. It reviews findings related to the specialization of neurotransmitter systems regulating human behavior and compares them to dimensions of temperament, a concept originally related to imbalances within chemical systems of the body. This convergence is presented in a functional perspective suggesting that consistent traits and their regulatory systems correspond to functional dynamical aspects of human actions (such as orientation, plasticity and endurance). The proposed neurochemical model, the “Functional Ensemble of Temperament” (FET), summarizes the overlap between temperament and neurophysiological models into a 4x3 matrix of traits that regulate endurance, speed of integration, orientation and also emotionality aspects of behavior. Each of these aspects appears to have different regulation depending upon the degree to which behavior requires probabilistic (analytic, mental), physical and/or verbal-social (habit based, more determined) actions. Unlike factor-analytic models looking for independent dimensions, the FET model suggests that regulatory systems underlying biologically-based traits are strongly interdependent due to contingent and feedback relationships.