Pathologic root resorption of permanent teeth, sometimes leading to premature tooth loss is a very frustrating clinical situation for the practicing dentist. This guide presents an overview of the various types of tooth root resorption, describes suspected resorptive mechanisms, and discusses diagnosis, management and associated prognoses of teeth with root resorption. Detailed sets of radiographs, several with post-therapy follow-up, demonstrate the clinical resorptive conditions. An easy to understand table differentiates the clinical, radiographic and recommended treatment differences between “Internal Resorption” and “External Resorption”. The fourth edition of this guide discusses the use of cone beam computed tomography as a definitive diagnostic tool in difficult cases.