A problem that is encountered in patients with low back pain is that a definite diagnosis is rare. The heart of our clinical dilemma is how best to diagnose those rare patients with severe disease as early as possible. The main reasons for diagnostic errors have been summarized in this chapter. Numerous guidelines have been developed to help in the approach to this difficult problem. However, the limitations of such an approach are highlighted. History and clinical examination are the best single test for diagnostic purposes and neither questionnaires nor computers can replace the clinician. However, the values of clinical abilities should not be overestimated. Finally, the clinical application of guidelines needs to be re-evaluated for each individual patient.