Fatigue is a nonspecific symptom because it can be indicative of many causes or con-ditions including physiological states such as sleep deprivation or excessive muscular activity; medical conditions such as chronic inflammatory conditions, bacterial or viral infections, or autoimmune illnesses; and psychiatric disorders such as major depression, anxiety disorders, and somatoform disorders (Manu, Lane, & Matthews, 1992). Fatigue may be caused by prescription medications such as antihistamines, drugs prescribed for insomnia, or chemotherapy drugs. Fatigue may also result from unhealthy lifestyles, such as frequent disruptions in the wake-sleep cycle, excessive alcohol or caffeine intake, and psychosocial stressors, or from the delayed effects of traumatic events (Manu et al., 1992). Finally, fatigue may be "unexplained," when none of the above causes is present or the causes cannot be determined. Chronic fatigue syndrome has recently received considerable attention, and it is an illness char-acterized by unexplained severe, persistent, disabling fatigue (Fukuda et al., 1994). Some describe fatigue in terms of physiological data or "objective" observations of decreasing muscle performance or decrements in work or performance. For example, fatigue has been defined as a failure to maintain a required force or output of power during sustained or repeated muscle contraction (Stokes, Cooper, & Edwards, 1988) or as time-related deterioration in the ability to perform certain mental tasks (Broadbent, 1971). In contrast to physiological fatigue, some have defined fatigue as a subjective self-reported feeling of fatigue. This feeling of fatigue is what people generally report when they seek medical treatment (Berrios, 1990). Fatigue is sometimes defined as "tiredness," feeling tired, being fatigued, feeling weak in part of the body, tired or lacking in energy, or experiencing "everything [as] an effort" (Cope, 1992, p. 273). Physiological definitions are more easily measured, but the subjective feeling of fatigue is not directly observable (Berrios, 1990). Further, this feeling of fatigue does not always correspond directly with physiological manifestations (Berrios, 1990). In response to its complex nature, most measures of fatigue have moved away from single questions (i.e., "Do you feel tired?") to a more multidimensional approach. These measures assess the effect of fatigue on daily activities, mental and physical aspects of fatigue, and other characteristics and related symptoms (Barofsky & West Legro, 1991; Chalder et al., 1993; Cope, 1992). Fatigue has also been described in terms of level of severity, level of impairment, physiological and psychological characteris-tics (physical vs. mental), and duration (Barofsky & West Legro, 1991; Cope, 1992). Whereas some researchers have proposed a categorical classification (i.e., absence or presence of fatigue), several research studies suggest that fatigue is best conceptualized on a continuum (Berrios, 1990; Loge et al., 1998; Pawlikowska et al., 1994; Wessely, 1998), with its variability reflecting degrees of severity (Pawlikowska et al., 1994). Despite these more recent attempts to compartmentalize fatigue, the major problem with such measures is their subjectivity.