Fatigue is one of the most prevalent and distressing symptoms of cancer, and is a common side-effect of many of the treatments available for the management of malignant disease. We critically assess the evidence for cancer-related fatigue and its treatment in adults. Little is known about the cause and mechanisms of fatigue, and research into methods of alleviating the condition has focused on treatment for anaemia and behavioural interventions, such as exercise, both of which are effective in reducing fatigue. Although research into the condition has increased considerably in the past decade, important gaps in knowledge remain.