Objective To calculate physician concentration levels for all U.S. markets using detailed data on integration and accountable care organization (ACO) participation. Data Source 2015 SK&A office‐based physician survey linked to all commercial and public payer ACOs. Study Design We construct three separate Herfindahl–Hirschman Index (HHI) measures and plot their distributions. We then investigate how prevailing levels of concentration change when incorporating more detailed organizational features into the HHI measure. Principal Findings Horizontal and vertical integration strongly influences measures of physician concentration; however, ACOs have limited impact overall. ACOs are often present in competitive markets, and only in a minority of these markets do ACOs substantively increase physician concentration. Conclusions Monitoring ACO effects on physician competition will likely have to proceed on a case‐by‐case basis.