The main goal of the endodontic treatment is to remove the infected tissue from the root canals, to disinfect their walls, to shape and hermetically seal them and to restore the tooth crown with a permanent material in order to maintain the functionality, the longevity and the aesthetics of the tooth. This is achieved by trepanating the tooth, removing the coronal part of the pulp, determining the length of the canal and by cleaning, shaping and filling the root canal. The endodontic therapy can be unsuccessful for various reasons. The therapist can make errors during diagnostic procedures such as the gathering of the anamnestic information or the conducting of clinical tests. However, the morphology of the tooth and its pulp space can also greatly affect the outcome of the therapy. During the cleaning and shaping of the root canal the following errors can occour: zipping of the root canal, canal transportation, stripping or lateral wall perforation and instrument separation. Furthermore, the errors that can happen during the filling of the root canals include: apical, coronal or lateral leakage, the determination of a wrong filling length, the existance of lateral accessory canals or the vertical root fracture during lateral condensation. Factors that can compromise the outcome of the endodontic treatment include intra and extraradicular infections and the presence of alien materials, cholesterol crystals, cysts and scar tissue in the periapical area. The endodontic treatment must be evaluated during regular recalls that must include a clinical check-up and a radiological overview.