In response to a long history of problems with defining and measuring fatigue, the University of Kansas School of Nursing established a Center for Biobehavioral Studies of Fatigue Management to facilitate the study of fatigue in diverse populations. The purpose of this article is to review past efforts to define and measure fatigue and the conceptual problems relevant to currently used measures of fatigue. Several distinct characteristics and corresponding measures of fatigue are identified and a definition and framework for the study of fatigue are discussed. Future research on fatigue must attend to the conceptual distinctions among various measures and the measures of fatigue most appropriate to the goals of a study.