Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is characterised by persistent unexplained fatigue resulting in severe impairment in daily functioning. CFS-like illnesses have been reported as early as the 19`h century. The lack of a recognisable, organic cause has lead to the illness and diagnosis being steeped in controversy, with researchers and patients disagreeing on the name given to the illness, the absence of pathophysiology, the contribution of psychological/emotional factors and the effectiveness of treatments such as cognitive behaviour therapy (CBT). Despite this debate, CBT has been the most researched treatment, with an evidence base that has shown that CBT improves fatigue in some patients. However, the current CBT treatment has some weaknesses and a significant proportion of patients do not respond. One such weakness is that the current CBT treatment model may not sufficiently address rumination and attention which have been identified as potentially important features of CFS/ME. Little is known about the factors that might drive the use of these strategies and how they might relate to the various facets of the illness. It has been shown that perseverative thinking strategies, such as rumination and attentional hypervigilence, in anxiety and depression can be influenced by metacognitive beliefs (beliefs an individual holds about their thinking). It may therefore be of interest to investigate this in CFS/ME. The main aim of the research was to generate preliminary evidence that metacognitive beliefs play a role in CFS/ME. The role of rumination and attention were explored by looking at the metacognitive strategies that people with CFS/ME use and the metacognitive beliefs they hold about such strategies. The research was a mixed methods design and consisted of a series of studies concerned with the development and validation of a questionnaire to measure metacognitive beliefs in CFS/ME (MB-CFQ): 1) a thematic analysis was conducted on transcripts from 10 semi-structured interviews with CFS/ME patients; 2) the results of the thematic analysis were used to design a questionnaire which was piloted in 104 CFS/ME patients and a principal components analysis was conducted; and. 3) a correlation analysis was conducted to provide some preliminary validation. The MB-CFQ showed high internal consistency and preliminary evidence of concurrent and construct validity. The questionnaire was used to investigate relationships between fatigue, metacognition and low mood. The data showed that, in CFS/ME, holding positive and negative metacognitive beliefs about the use of perseverative thinking strategies, including worry, rumination, and body monitoring (negative beliefs only), related positively to levels of fatigue (in particular mental fatigue), depression, stress, and anxiety. The results of this exploratory study now require further research to disentangle this observed relationship between metacognition, fatigue and mood.