Diagnostic errors are the most common errors in primary care. Diagnostic errors have been found to be the leading cause of malpractice litigation, accounting for twice as many claims and settled cases as medication errors. Diagnostic error is common, harmful, costly, and very critical to the patient-safety issues in health care. Diagnostic errors have received relatively little attention, however. Of what is known, diagnostic errors are an important source of preventable harm. Focused research in this area is highly needed because the causes of diagnostic errors are subtle and solutions are less obvious than for other types of errors. As opposed to medication errors, where the factors predisposing to their occurrence and the resultant preventive strategies are better defined, the relationship between factors influencing the diagnostic reasoning or decision making and a diagnostic error are not as clear. This may include any failure in timely access to care; elicitation or interpretation of symptoms, signs, or laboratory results, formulation and weighing of differential diagnosis; and timely follow-up and specialty referral or evaluation. The literature reveals that diagnostic errors are often caused by the combination of cognitive errors and system failure. Increased understanding about diagnostic decision making, sources of errors, and applying some existing strategies into clinical practice would help clinicians reduce these types of errors and encourage more optimal diagnostic processes.