Root canal treatment (RCT): an advanced technique for the treatment of endodontic and periapical diseases, in which the root canal is cleaned of necrotic and decayed material (infected or inflamed pulp, infected dentin, bacteria and their metabolites, etc.) by mechanical preparation and then tightly closed with a filling material. This is a surgical procedure to treat endodontic disease or to promote healing of periapical lesions. Root canal treatment is commonly known as nerve killing, which includes “removal of the pulp (i.e., nerve killing), creation and disinfection of the root canal, and slow enlargement of the thin root canal with a root canal file. The thin root canal should be enlarged slowly with root canal file, and then with the flushing of hydrogen peroxide and sodium hypochlorite, so that there is no residue of tissue debris and bacterial growth on the inner wall of the root canal, until the enlargement can reach the point of anti-inflammation and sterility, and the final step, which is the filling of the root canal, can be carried out. Indications I. Caries, hidden fracture, crown fracture, deformed central cusp, excessive wear, retrograde infection of periodontal disease, pulpitis caused by accidental pulp penetration and pulp necrosis that cannot preserve the living pulp; II. Continued development of pulpitis, failure of dry cadaver plastination treatment, apical lesions caused by retrograde infection of periodontitis; III. Infection of the apical pulp caused by physical or chemical reasons, such as: poor bedding temperature after filling pulpal irritation, current stimulation of different metals, apical infection caused by drugs during endodontic treatment; iv. degenerative changes of the pulp caused by unknown reasons, such as root canal resorption, pulp stone causing pathological changes of the pulp. The success rate of root canal treatment is clinically reported to be over 90%, which is the best treatment method to preserve the affected tooth. In general, root canal treatment is suitable for pulpitis, pulp necrosis, and various types of periapical inflammation, especially for teeth with residual crowns and roots, and root canal treatment is the only treatment method available for restorations that require pile crown restoration. Procedure I. Preparation before treatment: Through comprehensive medical history, clinical examination and radiographs, combined with the patient’s subjective symptoms, physical and mental state, the treatment plan is formulated. Full doctor-patient communication and detailed records. II. X-ray film: a total of three dental films, preoperative film, main cusp film and root filling completion film, are required to be taken. Painless technique and aseptic technique: The whole process of treatment emphasizes painless technique and aseptic technique to eliminate patients’ pain and fear and to prevent cross infection. The specific steps of root canal treatment: 1.Tooth preparation: including removing the decayed tissue, adjusting the occlusion, ligating or temporary crown restoration of the hidden tooth 2.Opening the pulp: it is required to uncover the full pulp chamber roof, so that the instruments can enter the root canal in a straight line without resistance and destroy the hard tissue of the tooth as little as possible. 3.Determine the location of the root canal opening. 4.Extraction of the pulp: the necrotic pulp is extracted gradually and deeply, and the non-necrotic pulp is extracted at once. 5.Determine the working length. 6.Root canal preparation: It is required to achieve a thorough cleaning of the root canal system, forming a form along the original root canal, with a certain taper, and ending at the apical base point. It is recommended to use the inverted method technique. 7.Rinsing: Lubrication and rinsing are used throughout the root canal preparation process. No pressure flushing is required. 8, Intracanal medication: calcium hydroxide is recommended, and the sealing time is 1-2 weeks. 9, root canal filling: the root filling is required to be filled tightly, 0.5-2.0mm from the root tip of the x-ray film.