Pathological external root resorption is a process by which the hard tooth substance is lost because of action of different local irritative or systemic factors. Six forms of external root resorption are described: surface external root resorption, inflammatory external root resorption, replacement external root resorption, invasive external cervical root resorption, ankylosis and transient apical breakdown. The objective of the study was to establish the prevalence of the pathological root resorption in a retrospective radiological study, to highlight radiological aspects of external root resorption and to describe optical coherence tomography (OCT) aspects in pathological root resorption. External root resorption prevalence in our study was 17%. Radiological exam was very useful to detect the form of root resorption but also to establish possible etiological factors. The study presents the radiological aspects of some forms of pathological external root resorption highlighted on radiography accompanied by radiolucency of the adjacent bone to resorption lesion (as in inflammatory external root resorption and in invasive cervical root resorption) or as changes of the root form, in which the missing part of the root is replaced by the bone tissue, without radiolucency (as in replacement root resorption). The comparison of the OCT aspects of the physiological root resorption in primary teeth with the OCT aspects of pathological root resorption in permanent teeth showed an obvious difference between the images of the OCT signal of the two types of resorption. The OCT signal from the cement is stronger in pathological root resorption, and the OCT signal from the dentin in teeth with inflammatory external root resorption showed a demineralized dentin with multiple heterogeneities, anfractuosities corresponding to resorption craters, with craze lines in subjacent layers of dentin.