Fatigability is defined as fatigue in the context of activity level. This FOA invites applications proposing to 1) investigate the role of specific bioenergetic factors in increased fatigability, reduced activity, and diminished sense of well-being in older persons; 2) test the effects of interventions targeted at such factors on performance capabilities, functional status, and other outcomes that relate to quality of life; or 3) develop and evaluate measures of fatigability applicable for observational and/or interventional studies.