Root canal therapy means cleaning the contaminated root canal system of all bacteria and their nutritional supply. To prevent recontamination of the system and to incarcerate any bacteria that prove resistant to the cleaning procedures, we then fill or obturate the root canal system. The now inactivate bacteria cease to be a problem or irritation to the surrounding vital tissues and providing the tooth and the supporting tissues remain healthy and do not succumb to more progressive decay or periodontal disease, research shows us that root treatments can last many decades. The latest instruments and techniques we now use enable us to access and prepare even the previously most inaccessible areas. The process is theoretically straightforward but can be time consuming if there are multiple canals with sharp curvatures or calcified channels and the root structure can vary from individual to individual making preparation more arduous. Provided stringent attention is paid to disinfection and the prevention of re-contamination during the root canal therapy, a high degree of success can be anticipated. First time treatments are more successful because the bacterial infection is limited to the tooth and has not invaded the supporting tissues.  Once these tissues become involved there is a slight but immediate reduction in the rate of success but the practice has come to specialise in re-root treating teeth that have failed to settle and are in danger of being condemned to extraction. Re-treatments are more complicated to disinfect and before we can begin the chemo-mechanical process accessing the root canal system can involve the removal of extremely difficult coronal and restorative materials.   The previous root canal filling materials then have to be removed and the root canal system fully instrumented and disinfected. This can be difficult and time consuming but providing there is sufficient remaining tooth structure, the gum condition is healthy and the quality of the re-treatment good, retention of the tooth is feasible and is significantly more advantageous than extraction. It has been practice policy to provide individual dental files to reduce the risk of cross contamination since 1989, long before it became compulsory and combined with the enhanced diagnostic facility of the cone beam CT scanner and microscope, the quality of root canal therapy is of the highest calibre.