This issue of Focus concentrates on the use of the Temperament and Character Inventory (TCI) to evaluate these fundamental questions about the effects of nature and nurture on the development of personality and psychopathology in a way that is useful for the promotion of health and well-being in clinical practice. The TCI measures both adaptive and maladaptive personality traits, which makes it particularly useful for evaluating alternative approaches to personality assessment and for differential diagnosis and treatment planning in general. Personality traits can be described in many different ways, and unfortunately the same name is often used to label different combinations of traits. For example, Eysenck distinguished only neuroticism, extraversion-introversion, and psychoticism, and these traits are closely related to three proposed for DSM-V (negative emotionality, introversion, and antagonism). The TCI measures were developed to better measure individual differences in psychobiological processes within the person that shape learning and development. They are compared with those of Eysenck and Zuckerman (2) in Table 1 and to those of Costa and McCrae in Table 2 (3). What is important for a clinician to know is that the same name does not always mean the same thing: different tests with the same name measure different things, as seen by examining the TCI correlates of the neuroticism measures of Eysenck, Zuckerman, and Costa. Fortunately, the TCI provides a system of measurement that encompasses other tests and has predictive validity as good as or better than that of other available tests (4).