Endodontic treatment is a highly successful procedure with reported success rates exceeding 85–95%. However, failures may occur due to persistent microbial infection, inadequate cleaning and shaping, missed canals, procedural errors, coronal leakage, root fractures, or host-related factors. Endodontic failure often manifests as persistent pain, swelling, sinus tract formation, or radiographic evidence of periapical pathology. Effective management requires accurate diagnosis, identification of etiological factors, and selection of appropriate treatment modalities. Contemporary management options include nonsurgical retreatment, endodontic microsurgery, regenerative endodontics, and extraction followed by prosthetic rehabilitation when necessary. Advances in cone beam computed tomography (CBCT), dental operating microscopes, ultrasonic instrumentation, and bioceramic materials have significantly improved treatment outcomes. This review discusses the causes, diagnosis, management strategies, clinical outcomes, and future directions in the treatment of endodontic failures. Evidence suggests that appropriate case selection and modern technologies can substantially improve prognosis and long-term tooth retention.