Under normal circumstances, teeth are resistant to the ravages of external invasive resorption (EIR), as odontoclasts cannot attach to a root surface that is covered by a precemental organic layer. However, when that layer is missing or damaged, which may be due to multiple factors, EIR may occur and move through the cementum and dentine. When this occurs there is usually a characteristic radiographic appearance in which the resorptive process moves longitudinally in the dentine without penetrating the organic predentine that protects the pulp. When superimposed by inflammation and/or bacterial contamination, the pulpal space may also be penetrated. This type of resorptive process is more commonly seen in cases of invasive cervical resorption; however it can occur more apically, in which case it would be considered invasive radicular resorption. This case report provides an in-depth assessment of invasive radicular resorption in a mandibular premolar using magnified, high-definition, visual evaluation and µCT sections to provide the clinician with a better understanding of this malady. Keywords: external invasive resorption, micro-CT, odontoclasts, radicular resorption, radiographs