The aim of this thesis was to examine the validity of the main concepts of metacognitive theory in a Turkish sample and set the stage for metacognitive research in Turkey from the clinical psychology perspective. In addition to this, research attention was focused on two important topics remained to be empirically validated in the metacognition literature: (1) the unique contributions of &ldquo / cognitive content&rdquo / versus &ldquo / metacognition&rdquo / to the prediction of anxiety and depression symptoms, and (2) the vulnerability function of metacognitions in the development of anxiety and depression symptoms. To achieve these generic aims of the study, a two-step research plan each of which has its own specific objectives was followed. Data for cross-sectional and prospective parts of the study were collected from Turkish and British non-clinical samples. In the cross-sectional part, mainly the independent contribution of metacognitions to pathological worry, obsessive-compulsive symptomatology, and anxiety and depressive symptoms above and beyond the contribution of cognitive content was evaluated. By doing so, also the relationship patterns between metacognitions and psychological symptomatology were revealed in the Turkish sample. Consistent with the recent burgeoning of research, the association between increased levels of metacognitions and increased levels of anxiety and depression was shown in the Turkish sample, as well. Moreover, metacognitive factors were found to be associated with the symptoms of anxiety and depression independently of the relevant cognitive content. In most analyses, metacognitions emerged as slightly stronger predictors of a given symptom dimension compared to the relevant cognitive content. In the prospective part, the causal role of metacognitions following stress in the development of anxiety and depression symptoms was examined. In the Turkish sample, higher levels of negative beliefs about worry predicted augmentation in anxiety and depression symptoms from Time 1 to Time 2. Besides, higher levels of lack of cognitive confidence interacted with higher levels of daily hassles to predict intensification of the anxiety scores. However, the British data did not support the causal role of metacognitions in the development of anxiety and depression symptoms. The statistical comparisons between Turkish and British samples indicated that the Turkish sample has a tendency to score significantly higher than the British sample on the metacognitive variables. Moreover, for all but one metacognitive factor, the interactions with cultural group (Turkish vs. British) were not significant in predicting psychopathology, indicating generalization of metacognitive theory to both the Turkish and British samples. Findings of this study were well in line with the metacognitive theory and discussed in the light of the relevant literature.