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Resorcinol-formaldehyde resin is a material used in endodontic therapy in many foreign countries. With immigration to the United States increasing, American dentists need to become familiar with resorcinol-formaldehyde therapy. It contains two potentially toxic components, formaldehyde (liquid) and resorcinol (powder). Zinc oxide or barium sulfate may be used for radiopacity. When 10% sodium hydroxide is added to the mixture, polymerization occurs, which can form a brick-hard red material that has no known solvent. Several variations in technique exist. The catalyst can be mixed in before insertion into the tooth, added after the mixture is inserted, or not used. Providers believe pulp tissue will be fixed and bacteria destroyed apical to the level of resorcinol-formaldehyde resin placement. Canals are frequently not instrumented or obturated to their full length. Few success–failure studies have been published and results are contradictory. Consequently, providers have little guidance regarding when to retreat or for predicting the difficulty of retreatment.