Evidence from autopsy studies indicates that thousands of patients die every year due to missed or delayed diagnoses, leading to diagnostic errors being termed the "next frontier" in patient safety. This Dutch study used trigger methodology (based on the classic Harvard Medical Practice Study) to analyze the epidemiology and underlying causes of diagnostic errors in a broad sample of hospitalized patients. Approximately 1 in 250 patients experienced a diagnostic error, most of which were considered preventable. The contributing factors primarily centered around knowledge-based errors and faulty information transfer between physicians—a problem noted in prior studies of diagnostic errors. A Patient Safety Primer discusses the heuristics that cause physicians to err in the diagnostic process and the system failures that lead to delayed or missed diagnoses.