Root canal therapy is a treatment procedure for tooth, pulp, and apical lesions. Root canal therapy is a treatment method to prevent the occurrence of periapical lesions or to promote the healing of periapical lesions by removing necrotic material from the root canal, performing proper disinfection, and filling the root canal to remove the adverse stimulation of the root canal contents to the periapical tissues. First, any cause (including caries, hidden fracture, crown fracture, deformed central tip, excessive wear, retrograde infection of periodontal disease, accidental pulp penetration) caused by pulpitis and pulp necrosis cannot preserve the living pulp. Secondly, apical lesions caused by any cause (including continued development of pulpitis, failure of dry cadaver plasticization treatment, retrograde infection of periodontitis). Third, physical and chemical causes of pulpal apical infections, such as: pulpal irritation caused by poor temperature of the cushion after filling, current stimulation of different metals, apical infection caused by drugs during endodontic treatment. Fourth, the degenerative changes of the pulp caused by unknown reasons, such as root canal resorption, pulp stone caused by the pathological changes of the pulp. The process of root canal therapy is to prevent the occurrence of periapical lesions or promote the healing of periapical disease by thoroughly removing the infected pulp as well as the infected dentin and toxic decomposition products through root canal irrigation, disinfection and tight filling of the root canal to isolate the bacteria from entering the root canal and reinfecting it. The success rate of root canal treatment is clinically reported to be over 90%, which is currently the best treatment method to preserve the affected tooth. In general, root canal treatment is indicated for pulpitis, pulp necrosis, and all types of periapical infections. In particular, root canal therapy is the only treatment option for teeth with residual crowns that require pile crown restoration in the prosthodontics department.