An endodontic flare-up is a complication of endodontic treatment which is defined as an acute exacerbation of asymptomatic pulpal or periapical pathoses after the initiation or continuation of root canal treatment. A flare-up is characterized by pain and/or swelling that may arise following initial debridement of the root canals or even after obturation. The severity of a flare-up varies according to the severity of the patient's pre-operative pathosis, signs and symptoms.Flare-ups can also be caused by inadequate debridement and disinfection of the root canals,microbial persistence after instrumentation, over-instrumentation, irrigant extrusion or apical extrusion of infectious debris during instrumentation. A thorough understanding of the etiology of flare-ups is required to prevent, properly diagnose and effectively manage patients presenting with pain or swelling.