The main objective of any root canal treatment of an infected teeth is to eliminate microorganisms that colonize the canal system. This disinfection is essentially based on a chemo-mechanical preparation, the endodontic instruments to shape the main canal, associated to the action of the irrigation solutions. The irrigation sequence should begin as soon as the access cavity is performed, during root canal shaping and at the end of the root canal preparation, in order to remove the smear layer generated by the action of the shaping instruments. This action is obtained by the combination of 2.5% sodium hypochlorite and an EDTA-based chelator (Ethylene diamine tetracetic acid). We will explore this whole process through a clinical case with cyst-like chronical periapical periodontitis.