A comprehensive understanding of the emotional mechanisms that motivate the symptoms of pediatric obsessive-compulsive disorder (OCD) is important for models of the etiology and treatment of the disorder. Existing models have emphasized the central role of fear, but recent research has highlighted that OCD may also be underpinned by incompleteness and disgust. However, whether incompleteness is relevant for the understanding of pediatric OCD has not been examined and no studies have conjointly examined fear, incompleteness, and disgust in clinical samples. The present thesis concerned itself with the degree to which fear, incompleteness, and disgust (a) could be validly assessed in youth with OCD and anxiety disorders; (b) discriminated pediatric OCD from pediatric anxiety disorders; (c) underpinned the main symptom dimensions of pediatric OCD; and (d) predicted treatment outcome for youth with OCD. Youth seeking treatment for pediatric OCD and anxiety disorders (assessed via structured diagnostic interviews) completed self-report and interview-based measures of trait (everyday experiences) and state (direct emotion involvement in symptoms) levels of fear, incompleteness, and disgust. Non-clinical youth were recruited from local schools and completed the same trait-level measures. Results showed that both children and adolescents were able to comprehend the concept of incompleteness and to report on whether this emotion, as well as fear and disgust, was part of their general emotional responses and in their symptoms of OCD and anxiety disorders. Trait-level fear and disgust were elevated in youth with OCD and anxiety disorders when compared to non-clinical youth, while incompleteness was only elevated in youth with OCD. When examining dimensional associations between self-reported trait-level emotion and self-reported symptom severity, fear was positively associated with OCD, anxiety, and depression, while incompleteness was uniquely associated with OCD, and disgust uniquely associated with anxiety. Regarding the direct involvement of these emotions in the symptoms of OCD and social anxiety disorder and generalized anxiety disorder, fear was again shown to be related to both OCD and anxiety disorders, incompleteness specifically related to OCD (particularly symmetry-related OCD), and disgust specifically related to contamination-based OCD. Youth with OCD symptoms characterized by high levels of incompleteness and disgust evidenced a poorer response to OCD treatment. Overall, the present thesis suggests that additional studies are warranted that examine the relationship between incompleteness and disgust in relation to the heterogeneity and treatment of pediatric OCD. Further, to better outline factors specifically related to the development and maintenance of pediatric OCD, theoretical models should more clearly account for emotion-related motivators other than fear.