The aim of this thesis was to consider the role of appraisals of intrusive thoughts in the development of Obsessive-Compulsive Disorder. A narrative literature review explored the hypothesis that 'normal' intrusive thoughts lie on a continuum with clinical obsessions. The review discussed previous research on intrusive thoughts in nonclinical samples and drew comparisons with characteristics of clinical obsessions. An internet-based empirical investigation employed a randomised controlled trial design in order to test the effectiveness of an intervention based on normalising information in reducing problematic meta-cognitive beliefs. A large sample (N = 148) of young adults (aged 18-20 years) was screened in to the study based on high levels of problematic meta-cognitive beliefs. Participants completed questionnaire measures of meta-cognitive beliefs, obsessive-compulsive symptoms, reactions to intrusive thoughts and experiential avoidance. Participants completed an interactive quiz based on normalising information (experimental condition) or pet information (control condition). Significant reductions in problematic meta-cognitive beliefs and experiential avoidance were observed in both conditions, thus no additional benefit of normalising information was indicated. The implications of these findings are discussed in the context of the potential normalising effects of symptom monitoring. Overall this thesis supports the comparison of 'normal' intrusive thoughts and obsessions and suggests that negative appraisals, such as problematic metacognitive beliefs, may not be the only defining factor in the development of Obsessive-Compulsive Disorder.