Competition in Health Care Markets11We wish to thank participants at the Handbook of Health Economics meeting in Lisbon, Portugal, Pedro Pita Barros, Rein Halbersman, and Cory Capps for helpful comments and suggestions. Misja Mikkers, Rein Halbersma, and Ramsis Croes of the Netherlands Healthcare Authority graciously provided data on hospital and insurance market structure in the Netherlands. David Emmons kindly provided aggregates of the American Medical Association's calculations of health insurance market structure. Leemore Dafny was kind enough to share her measures of market concentration for the large employer segment of the US health insurance market. All opinions expressed here and any errors are the sole responsibility of the authors. No endorsement or approval by any other individuals or institutions is implied or should be inferred.