Because hospitals and health systems sponsored the majority of new accountable care organizations (ACOs) from 2010 to 2015, they influenced priorities and strategies of the policies designed to drive ACO adoption. In recent years, however, the majority of new ACOs have been sponsored by physician groups. This shift means that policies need to be developed with the characteristic strengths and weaknesses of physician-led ACOs in mind. Using data from the Leavitt Partners ACO database, we analyzed the types of providers becoming ACOs over time to look at their numbers and market potential. Because the market potential for further growth of physician group-led ACOs is much stronger than for hospital- or health system-led ACOs, policy makers need to create programs and policies that facilitate physician-led ACOs' success by helping them develop the capacity to take on risk, finance investments in high-value healthcare, and partner with other organizations to provide the full spectrum of care.