Fatigue is a commonly experienced symptom, which may be a component of virtually any disease and can have a psychological, physical, or mixed origin. Nurses need to understand the onset, duration, and progression of fatigue to intervene successfully with the cancer patient adapting to diagnosis and treatment. While the literature is an important source of information, results of research studies must be critically interpreted before proceeding with practice guidelines based on research findings. A critical appraisal of the research literature investigating the problem of fatigue in individuals with cancer was conducted. There is strong evidence to suggest that fatigue is a prevalent problem among cancer patients receiving chemotherapy and radiation therapy. However past research has been limited by methodological problems. Typically, studies fail to include a control group, do not control for possible confounding variables, and have restricted measurement to unidimensional scales with limited reliability and validity. While several correlates of fatigue have been postulated, research to date has found no consistent relationships among such correlates as weight loss, anemia, or psychological distress. This article reviews what is currently known about fatigue in the cancer patient and how future research could be designed to improve on past measurement and sampling problems.