Fatigue is often related to cancer, and is related to its treatment. Fatigue is the most commonly reported side effect of cancer treatment. It differs from fatigue induced by other causes, because doesn't decrease after period of rest. The fatigue reported by cancer patients is usually described as an unusual, excessive, whole-body experience that is disproportionate or unrelated to activity or exertion and is not relieved by rest or sleep. Cancer related fatigue is multidimensional, subjective experience that has profoundly negative effect on patients' quality of life. Fatigue today is the most commonly reported symptom, for some patients the most unpleasant symptom, and the most side-effect in connection with cancer and/or its treatment. The prevalence of fatigue in patients receiving anti-cancer treatment has been estimated to be more than 80 per cent. The fatigue can be pervasive: patients report that fatigue begins with treatment, continues during the course of chemotherapy or radiation treatment, and declines somewhat--but frequently sustains at a higher-than-baseline rate-after treatment is over. It may also persist for several years even in patients with no apparent disease. Despite of the prevalence of fatigue and its profoundly negative effect on patients quality of life, little is known about the specific mechanisms that underline fatigue in cancer patients or how to prevent it and treat it effectively.