Diagnosis is a foundational process of the practice of medicine. The correct and timely identification of a health condition is a first step in ensuring that it is properly treated or managed. Most people will experience at least one diagnostic error in their lifetime, sometimes resulting in severe patient harm and up to 80% of all harm caused by delayed or misdiagnosis could be preventable. Deficits in health system design and governance, clinical environments, and individual provider competencies can drive poor diagnostic outcomes. This brief introduces OECD work to assess key drivers and barriers of diagnostic safety and estimate the economic impacts of poor diagnostic safety practices on health systems.