An endodontic flare-up is a complication of endodontic treatment which is defined as an acute exacerbation of asymptomatic pulpal or periapical pathology 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 signs and symptoms. Flare-ups can also be caused by inadequate debridement and disinfection of the root canals, microbial persistence after instrumentation, over instrumentation, irritant extrusion or apical extrusion of infectious debris during instrumentation. The etiology of flare-up is multifactorial. Even though it has been demonstrated that a flare-up has no significant influence on the outcome of endodontic treatment, its occurrence is extremely undesirable for both patient and the clinician and can undermine the clinician- patient relationships. Therefore, clinician should employ proper measure and follow appropriate guidelines in an attempt to prevent the development of interappointment severe pain and or swelling.