Fatigue is one of the most commonly symptoms reported by patients. There are many causes of pathological fatigue including mental and physical, both neurological (central and peripheral) and non-neurological diseases. Among the last ones, cancer, infections and drugs are factors associated with fatigue, although the mechanism of how non-neurological diseases cause fatigue is not clear. Evidence exists to suggest that patients who have been critically ill continue to face a multitude of physical, psychological and social difficulties in the long term after discharge. As far as fatigue is concerned, greater than 50% of Intensive Care Unit survivors overall reported lowered energy levels and fatigue in the first year after discharge.