More than 55 percent of the U.S. population now lives in an area served by an accountable care organization (ACO). But the spread of this model of delivering and paying for care will not ensure its success. In a new blog post, Elliott S. Fisher, M.D., William L. Schpero, and Janet M. Corrigan of The Dartmouth Institute for Health Policy and Clinical Practice outline policies for standardizing measurement of ACOs’ performance to determine whether they are meeting their goal of better care at lower cost.