 Selection and insertion of an endotracheal tube, ETT, of appropriate size for airway management during general anesthesia in pediatric patients is very important. A very small ETT increases the risk of inadequate ventilation, air leakage, and aspiration, whereas a very large ETT may cause serious complications including airway damage, post-intubation croup, and, in severe cases, subglottic stenosis. Although the pediatric larynx is conical, the narrowest part, the remaglottidus, is cylindrical in the antroposterior dimension, regardless of development, and the cricoybring is slightly elliptical. A cuff ETT reduces the number of endotracheal intubation attempts, and if cuff pressure can be maintained within a safe range, the risk of airway damage may not be greater than that of an ETT without cuff. The age-based formula suggested by Cole, age-slash-4-plus-4, has long been used to select the appropriate ETT size in children. However, this formula is not always accurate, so various alternative methods for estimating the ETT size. This article was authored by Sian Park, Sangwook Shin, Hygiene Kim, and others. We're article.tv, links in the description below.