Measurement in patient safety is challenging and requires consensus to determine the presence of specific problems. In light of recommendations from the Improving Diagnosis in Health Care report, this commentary reviews five purposes for measuring diagnostic errors: determining the extent of diagnostic errors; understanding their underlying causes; measuring whether interventions work; skill assessment for training and education; and tracking performance. The authors describe how achieving each of these goals will require distinct strategies. A PSNet interview with Hardeep Singh discussed issues of measurement in diagnosis.