This thesis focuses on cognition in Chronic Fatigue Syndrome (CFS). Many patients report difficulties such as `fuzzy' thinking, poor memory, reduced attention, and slowness of thought. Whilst much of the earlier literature presented conflicting results, more recently the theory that there is slowed processing has been suggested. Standard neuropychological tests have been used and been helpful in the description of this slowing, however an explanation for this slowing has not been postulated. This thesis proposes a slowed processing theory of cognition in CFS focusing on representational weakness and global reductions in cortical activity as possible mediators of slowing. Sixty-eight CFS patients (tertiary care clinic attendees) and 63 healthy controls participated in the study. They completed standard neuropsychological tests from the Weschler Memory Scale-R and measures of comorbid symptomatology, specifically the Hospital Anxiety and Depression Scale, The Fatigue Scale and the Profile for Fatigue Related Symptoms. They also completed a battery of tests designed to assess whether CFS patients had slowed performance; whether these problems could be attributed to representational weakness; whether there were differences in CFS and control participants' performance on perceptually and conceptually processed tasks, and tasks requiring conscious and non-conscious processing. The role of non-novel versus novel stimuli and interference is also discussed. The The results suggested that CFS patients' recall was worse than control participants' on the following measures: Paired Associate Learning - hard pairs, Logical Memory, and explicit memory. They were slower than controls for all levels of processing graded from perceptual to conceptual, and for semantic judgements of word pair relatedness. The results are discussed as support for a theory of cognition in fatigue which is dependent on 2 factors; firstly, representational weakness and secondly global slowing of cortical activity. It is proposed that these two factors interact, and the performance of CFS patients on what may initially appear to be similar tests can be quite discrepant.