Diagnostic errors remain relatively understudied and underappreciated. They are particularly concerning in the intensive care unit, where they are more likely to result in harm to patients. There is a lack of consensus on the definition of diagnostic error, and current methods to quantify diagnostic error have numerous limitations as noted in the sentinel report by the National Academy of Medicine. Although definitive definition and measurement remain elusive goals, increasing our understanding of diagnostic error is crucial if we are to make progress in reducing the incidence and harm caused by errors in diagnosis.