This thesis has been prepared in a paper based format and includes an empirical paper, a systematic review and critical reflection. As a whole, the thesis focuses on the importance of sleep in Chronic Fatigue Syndrome (CFS). The systematic review is prepared for submission to 'Sleep Medicine Reviews'. The empirical paper is prepared for submission to the 'Journal of Consulting and Clinical Psychology'. Paper 1 is a systematic review and narrative synthesis of the current evidence for the effectiveness of Cognitive Behavioural Therapy (CBT) and Graded Exercise Therapy (GET) on sleep in CFS. Eight studies were found and their methodological quality varied. To understand heterogeneity in findings, information regarding intervention delivery, including the presence of sleep management components, methodology and sleep outcome measures was extracted and synthesised. We conclude that GET can improve sleep, when delivered by experienced therapists in outpatient settings. The evidence for CBT on sleep is limited, moreover, at present we know little about the effectiveness of adding sleep management components to interventions. We suggest that sleep outcomes used previously have not been sufficiently comprehensive and sensitive to measure change in sleep difficulties experienced in CFS. Implications for further research are discussed. Paper 2 presents an empirical study examining the relationship between sleep and fatigue in CFS using a daily diary approach. Sleep was measured objectively using actigraphy, and subjectively, using sleep diaries, in order to test which parameters better predict next-day fatigue. We also examined whether negative mood could mediate these relationships and whether subjective sleep variables were predicted by pre-sleep arousal. Using multilevel modelling, we found that subjective sleep, and not objective sleep, predicted next-day fatigue and these relationships were partially mediated by negative mood on waking. Pre-sleep cognitive and somatic arousal predicted subjective sleep variables including sleep efficiency and quality. Based on these findings, we suggest that interventions targeting subjective sleep, such as CBT for insomnia, may be useful in improving experiences of fatigue in CFS. Paper 3 is a critical reflection on the systematic review and empirical research, and on the process as a whole. Strengths and weakness of Paper 1 and Paper 2 are discussed, in addition to consideration of their contribution to wider research, and clinical practice.