Measurement and assessment, occurring within both research and clinical service contexts, typically involve an imbalance of power between professionals and persons with disabilities. Power is evidenced in who controls decisions about measurement and whose perspective--the subjective values of the measured person or the objective or normative values of the measurer--is given primacy. The consequences of this imbalance are discussed with respect to both sides of the power relationship. For clinicians, evaluators, and researchers, who typically hold most, if not all of the power in measurement, the process may produce data that meet the highest professional standards. However, the utility of such data is limited in addressing many purposes. For research participants and service recipients, who typically have little if any control of measurement, the measurement process may be disempowering, because measures focus on areas of life that may be of little relevance to what they see as important. In effect, both sides lose, to the degree that resulting data are less revealing than would be the case in a more balanced power relationship. Methods are discussed for reducing power imbalances to improve the utility and efficacy of measurement.