This chapter presents a case of symptomatic apical periodontitis associated with a previously inadequate root canal treatment. Short obturation in all the canals and suspected unusual anatomy of the distal root of a mandibular molar. Both distal and mesial roots associated with periapical radiolucency. Failure to negotiate the full length of the root canal system during the endodontic process might result in inadequate cleaning and shaping, leading to persistence of the disease. The chapter also emphasises on evaluating the restorability and prognosis of the tooth in order to make a proper case selection for providing retreatment. It discusses the various factors that can help the clinician to make a case for nonsurgical or surgical retreatment. The chapter contains concrete evidence-based and practical techniques accompanied by full-colour, self-explanatory clinical photographs taking the reader through a journey of successful management of the failed clinical case.