]This study is a continuation of the IOM Quality Chasm Series, which focuses on assessing and improving the quality and safety of health care. The charge to the committee was to synthesize what is known about diagnostic error as a quality of care challenge and to propose recommendations for improving diagnosis. According to the report, diagnostic errors—inaccurate or delayed diagnoses—persist throughout all settings of care and continue to harm an unacceptable number of patients. The committee concluded that improving the diagnostic process is not only possible, but also represents a moral, professional, and public health imperative.