Objective To provide the first nationally based information on physician practice involvement in ACOs. Data Sources/Study Setting Primary data from the third National Survey of Physician Organizations (January 2012–May 2013). Study Design We conducted a 40‐minute phone survey in a sample of physician practices. A nationally representative sample of practices was surveyed in order to provide estimates of organizational characteristics, care management processes, ACO participation, and related variables for four major chronic illnesses. Data Collection/Extraction Methods We evaluated the associations between ACO participation, organizational characteristics, and a 25‐point index of patient‐centered medical home processes. Principal Findings We found that 23.7 percent of physician practices (n = 280) reported joining an ACO; 15.7 percent (n = 186) were planning to become involved within the next 12 months and 60.6 percent (n = 717) reported no involvement and no plans to become involved. Larger practices, those receiving patients from an IPA and/or PHO, those that were physician‐owned versus hospital/health system‐owned, those located in New England, and those with greater patient‐centered medical home (PCMH) care management processes were more likely to have joined an ACO. Conclusions Physician practices that are currently participating in ACOs appear to be relatively large, or to be members of an IPA or PHO, are less likely to be hospital‐owned and are more likely to use more care management processes than nonparticipating practices.