This brief review summarizes key epidemiological and experimental evidence concerning relationships between chronic physical activity and feelings of energy (vigor, vitality) and fatigue. The epidemiological studies show a positive association between the amount of typical weekly physical activity reported and the frequency with which people report feeling energetic. The randomized controlled experiments show that 10-20 wk of exercise training is associated with an increase in the frequency and intensity of feelings of energy among fatigued people with medical conditions. The results of longitudinal studies with non-fatigued, healthy adults are mixed. Overtraining by athletes is associated with increased intensity of feelings of fatigue. Additional well-controlled investigations into relationships between physical activity and feelings of energy and fatigue, especially among sedentary and fatigued individuals, are warranted given the available evidence and the importance of these moods to health, work productivity, and quality of life.