External apical root resorption (EARR) is frequently  observed during orthodontic treatment; it can require a  pause or termination of the treatment in the patient’s best interest. If the root length is reduced severely, it also affects the decisions regarding other dental procedures, for instance, prosthetic restorations. Thus, EARR should be assessed and managed properly throughout orthodontic treatment and its etiology should be fully understood. Although more detailed information of roots can be obtained with the latest technology, it is difficult to clarify which diagnostic or treatment factors cause EARR due to its multifactorial nature. Therefore, it is important to keep abreast of the latest evidence to avoid unnecessary EARR. This article will provide the readers with the information about EARR during orthodontic treatment.