Conventional endodontic therapy enjoys a high success rate; however, root canal treatment in cases of pulpal necrosis and cases with procedural errors will occasionally fail. Retreating such cases is often a challenge for clinicians. This chapter discusses both the diagnosis of retreatment needs and steps to regain access to the previously treated root canal system. In cases where satisfactory retreatment is impossible and healing cannot be expected, strategies for further decision-making are presented.