Progress in patient safety has been limited by a lack of reliable measures. This problem is compounded in ambulatory care, as most existing metrics have sought to measure safety in hospitalized patients. This systematic review identified 182 published safety measures in primary care and categorized them according to Donabedian's triad and the safety target. The majority of metrics sought to measure safety in medication management, with laboratory testing and care coordination among the other types of safety issues being assessed. The authors note several limitations with the metrics they identified—most had not been validated, and there were no published measures identified for diagnostic error (despite increasing evidence that such errors are common in the outpatient setting). A PSNet interview discusses the challenges of measuring and improving safety in the ambulatory care environment.