The outcome of orthodontic treatment may be jeopardized by severe apical root resorption, induced by orthodontic forces. All potential predisposing factors, systemic as well as local, must be considered before treatment begins. To evaluate the hypothetical influence of systemic factors, a detailed medical history should be recorded. Clinical intraoral examination should include standardized periapical radiographs taken with a film holder. The following factors should be evaluated: existing root resorption, including that due to disturbances of eruption, apical root form, invagination, agenesis, and short root anomalies. In patients with several predisposing factors, the treatment plan should be modified to take into account the risk of resorption. Modifications may include a shorter treatment time, less force, and a limited treatment goal. During treatment, a follow-up visit at 6 months should include periapical radiographs of all maxillary and mandibular incisors. The absence of radiographic signs of root resorption at 6 months indicates minimal risk of severe resorption at the end of treatment, whereas resorption at this stage of treatment indicates a risk of progressive resorption as treatment proceeds. The risk may be reduced by temporary suspension of active treatment for 2 to 3 months. After completion of treatment, radiographic examination is mandatory. If there is minor or moderate resorption, no further action is necessary. In severe resorption, where the foreshortened root remaining is no longer than the crown, there is a risk of increased tooth mobility. World J Orthod 2003;4:19–30.