The purpose of this research project was to further the understanding of the construct of fear of emotion, its measurement, its relationship to anxiety disorders and depression, and how it changes over treatment and follow-up with CBT alone or combined with medication in a community mental health centre. Anxiety and depressive disorders cause considerable distress and impairment. Understanding the mechanisms that maintain these disorders is important to optimise the effectiveness of available treatments. Fear of emotion is postulated as a transdiagnostic construct that maintains anxiety and depression and is amenable to treatment. Two studies were carried out; in the first, data from 652 adult patients who had completed treatment were evaluated to determine the association between fear of emotion and symptoms of anxiety and depression, the change after CBT in those who were medicated and those who were not, and those who achieved clinically significant change. Fear of emotion was a weak to moderate predictor of depression and anxiety symptoms independent of medication status. Subscale scores explained a greater proportion of variance of symptoms than total scores, and suggested some specificity in differentiating between the symptoms and the fear of specific emotions. Fear of emotion decreased over treatment regardless of whether patients were medicated or not. There was a trend for a greater proportion of unmedicated patients to achieve clinically significant change. In the second study, 41 patients who completed treatment were assessed at six-month follow-up. Self-reported fear of emotion following CBT treatment predicted severity of anxiety and depression symptoms at follow-up, regardless of medication status. Enhancements to current treatments should continue to target fear of emotion