When a clinician is confronted with an apical radiolucent lesion he must follow an organized thought process and diagnostic technique prior to therapy. This technique employs the collection of data, called the diagnostic data base, and radiographic data. The diagnostic data base and radiographic data combined with the results of the vitality tests will provide the clinician with an appropriate working diagnosis. If the data support a diagnosis of radiolucent lesion associated with a nonvital tooth then endodontic therapy should be provided and the patient should be seen on a routine follow-up basis. Dr. H. M. Worth has most elequently summarized the responsibilities of the clinician as follows: "The endodontist cannot content himself with sufficient knowledge to deal with apical infections only, for he encounters a great number of diseases; it is necessary for him to be reasonably well informed on the radiographic appearances of most of the abnormalities and diseases that occur in the jaws."