Root resorption is a process of losing cement, dentin, and alveolar bone. Pathological resorption implies the loss of the hard dental tissue. The resorption etiology is multifactorial but is mostly connected to the infections of pulpo-periodontal complex, traumas, and too strong orthodontic forces. Physiological resorption causes the root resorption of deciduous teeth. External resorption is much more common than the internal one and its advancement can endanger tooth integrity. Orthodontic therapy can cause root resorption during the processes of teeth movement and bone remodeling. If the conditions of the therapy are optimal and the appropriate strength of orthodontic forces are applied, the risk of developing resorption is minimal. Clinically speaking, based on the number of affected teeth, resorption can be localized and generalized. The level of resorption is determined based on the reduction of the initial root length. Therefore, we can differentiate between the initial resorption which is less than 2 mm, the advanced resorption which is 2-4 mm, and the strong apical resorption which reduces the length of the teeth for 1/3 of its initial length. Early diagnosis of the external and internal resorption can help to retain the tooth. It is important to conduct the appropriate therapy and remove the source (forces, infections, chemical causes). The correct diagnosis is based on medical history, clinical examination, and radiological analysis, and further therapeutic guidelines depend on how affected the dental tissue is.