Chronic fatigue syndrome (CFS) is a chronic debilitating condition affecting both physical and mental functioning. It was first quoted as a 'new disease' spreading in the developed countries. It bei:ame a major issue by doctors, professionals and the media for the past 15 years. CFS was not only affecting the adults but childhood fatigue has also been noted. The CFS patients commonly described themselves to be perfectionists, highly driven, energetic U1d motivated before the condition started. Studies have been focused on the definition, diagnosis and management of CFS. However, the understanding of CFS and what cause it is stili unclear and controversial. Thus the aetiological factors of CFS are reviewed in this article. KEYWORDS; Chronic fatigue syndrome (CFS), Aetiology, Psychiatric disorders, Viral infection, Immunology