Emotional disorders of anxiety and depression have been recognized as the most prevalent mental disorders in children and adolescents. High rates of co-morbidity between anxiety and depression have been reported in both adults and youths. Clark and Watson (1991) proposed a tripartite model to explain both the overlap and distinctiveness of anxiety and depression. In that model, negative affectivity (NA) is experienced in both anxiety and depression; lack of positive affectivity (PA) is specific to depression; and high physiological hyperarousal (PH) is unique to anxiety. Two decades of research among adults have supported the utility of the tripartite model. However, its applicability to children and adolescents has been studied less, and the results have been controversial. This controversy could be caused by two main issues. First, early youth research on the tripartite components has had to borrow adult measures to represent the tripartite constructs. Second, even after child-specific measures for the tripartite constructs were developed, relatively few studies have examined all three tripartite components together. In particular, PH has received less attention than NA and PA. The present study addresses these issues. First, this study assesses all three components of the tripartite model among Chinese children and adolescents in Hong Kong schools. A deliberate attempt is made to use measures of the tripartite constructs that are specific to children in order to improve the construct validity of the measurements. Finally, special attention is paid to the differential utility of the PH construct. Results partially supported the Clark and Watson’s tripartite model (1991). NA is a common component to both anxiety and depression. However, both PA and PH are also common components to both anxiety and depression. In addition, high correlations were found among anxiety, depression, and PH. These results tend to suggest more intricate relationships among anxiety, depression, and somatic symptoms. To conclude, this study suggests an alternative transdiagnostic approach to the tripartite model, which emphasizes clustering of anxiety, depression, and somatic symptoms. / published_or_final_version / Clinical Psychology / Doctoral / Doctor of Psychology