The frequent occurrence of fatigue and disturbed sleep in cancer survivors and the negative effect of these symptoms on quality of life and clinical outcome underscore the need to identify mechanisms that cause cancer-related fatigue, with a view toward developing more effective treatments for this problem. Human studies of fatigue and disturbed sleep are limited by high inter-individual genetic and environmental variability, difficulties with behavioral or reporting compliance, and the subjective nature of the problems. Although animal models also must overcome the barrier of assessing fatigue and sleep disturbance in the absence of obvious objective clinical markers, animal studies are easier to control and standardize than are studies of people. Moreover, animal models are crucial to the identification and understanding of underlying disease mechanisms. This review describes the need for, the feasibility of, and several possible approaches to measuring fatigue in animal models of cancer and to relating such measures to disturbed sleep, immune function, and other potential mechanisms. Developing and using animal models to better understand fatigue and disturbed sleep related to cancer and its treatment has an enormous potential to expand the knowledge base and foster hypotheses necessary for the future development and testing of interventions. Copyright 2008 by the American Association for Laboratory Animal Science.