Results showed that the reactive temperament factor of negative affectivity was positively associated with internalizing and externalizing symptoms, whereas the regulative temperament factor of effortful control was negatively related to such symptoms. In particular, the combination of high negative affectivity and low effortful control was associated with high symptom levels. Evidence was also obtained for specific relations between reactive and regulative low-order temperament traits and internalizing and externalizing symptoms. Fear and low attention control were more clearly associated with internalizing symptoms, whereas anger/frustration, and low activation and inhibitory control were more to externalizing symptoms. Fear appeared to be more relevant for internalizing symptoms, whereas anger/frustration was found more important for externalizing symptoms; lack of attention control was particularly related to internalizing symptoms, whereas problems with activation control and inhibitory control were clearly associated with externalizing symptoms. Finally, the temperament factors of negative affectivity and extraversion/surgency demonstrated the predicted associations with their Eysenckian counterparts of neuroticism and extraversion, respectively. Extraverted youths with externalizing problems more overtly express their negative emotions and disturbed behavior than introverted youths with such problems. High levels of negative affectivity and low levels of effortful control, particularly the combination of these two temperament characteristics, make children and adolescents prone to develop psychological disorders. Stress provoking stimuli and situations elicit negative emotions in youths with high levels of negative affectivity. Those with low levels of effortful control act on the basis of their reactive temperament as they cannot regulate these emotions, and are vulnerable to develop psychological problems. However, youths with high levels of effortful control are capable of overriding these maladaptive reactions, and instead, deploy adaptive coping responses, which serve to protect them against the development of psychopathology. Limitations of the study and suggestions for future research are discussed. Tables, figures, references