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Medically, nonexplained fatigue is a significant clinical and public health concern. However, the construct fatigue is not easily defined, and this is reflected in the large number of different scales used to measure fatigue. In this chapter, we define fatigue using the sustained stress activation theory. It is proposed that sustained stress, with a lack of sufficient recovery periods, is characterized by a decline in the ability to concentrate, be energetic, and sleep. Based on this definition, the brief fatigue syndrome scale (BFSS) was created using visual analogue scales (VAS) to rate self-assessed energy, ability to concentrate, and quality of sleep. The BFSS consisted of one factor, with a Cronbach΄s α of >0.70. The validity and sensitivity of the scale was assessed by following a group of fatigued patients undergoing a 1-year lifestyle recovery program. In addition, a reference group of healthy controls was followed over the same period. Both groups responded regularly to a survey containing the BFSS. Blood samples were also collected during the first 6 months. The BFSS scores decreased significantly as the clinical conditions of the fatigued participants improved. Furthermore, self-rated mental energy, assessed by a separate and validated five-item Likert-type scale, improved with decreasing fatigue. Participants scoring above the proposed cut-off point of 8 for depression on the hospital depression and anxiety scale had significantly higher BFSS scores. BFSS scores were also higher among subjects scoring in the burn-out range on the Shirom-Melamed scale. Decreased fatigue over time was related to improved/ increased serum levels of testosterone. It is suggested that the cut-off point for fatigue vs. nonfatigue should be set at 40%. The BFSS is suggested as a valid and quick instrument to assess fatigue. [ABSTRACT FROM AUTHOR]