Introduction: Flare-up is defined as moderate to severe postoperative pain and/or swelling, beginning 12 to 48 hours after treatment and lasting for, at least, 48 hours. Aim: Evaluate which of the approaches, single session or multiple session, presents lower probability of appearance of Flare-up. Materials and Methods: The literature search was based on the b-On search engines, National Library of Medicine, PubMed Database, using, in different combinations, the terms: "flare-up", "endodontics", "postoperative pain" and "number of sessions", with the time limit between 2010 and 2019. Meta-analyzes, bibliographic reviews and randomized controlled trials were included. Excluded were articles whose purpose deviated from the main theme and those whose methodology of research was incomplete. Results: Seventeen articles were selected, 6 of which are systematic or bibliographical reviews and 11 are clinical studies. Conclusions: According to the literature reviewed, the number of sessions chosen to address the Non-Surgical Root Canal Treatment (NSRCT) is not a predisposing or precipitating factor of the occurrence of Flare-up. Nevertheless, the literature on the subject is still scarce and presents some defects. Further studies are needed to unambiguously establish the association or not with the number of NSRCT sessions. Clinical Implications: Knowledge of the factors that may be associated with the development of Flare-up aids the Dentist in the selection of the best approach to treatment, knowing that the number of sessions does not appear to be a prominent variable as long as the antimicrobial efficacy of the treatment is ensured. Palabras clave: flare-up, endodontics, postoperative pain, number of sessions