Several research tools help to unravel ECR pathology and bring information together in a 3D understanding of ECR dynamics. The synergistic use of surgical microscopy, digital radiography, Cone Beam CT, Micro-CT, Nano-CT, scanning electron microscopy, hard tissue and soft tissue histology and immunohistochemistry helps to visualize the numerous morphological and histological changes. The formation of vascularized bonelike tissue as a substitute for resorbed enamel, cementum, dentin and the pericanalar root resorption resistant sheet (PRRS) is extremely complex. Also the bone turnover of the bonelike tissue can be proven and is a regular process occurring in hard tissue biology. The formation of intracanalar and intrapulpal reaction calcifications indicate a chronic pulp irritation. The portal(s) of entry and portal(s) of exit for ECR can be specified. The etiology of ECR is multifactorial. Most common causes are hypoxia inducing insults: walking bleach technique, orthodontic treatment, cementum damage induced by extraction of neighboring teeth, parafunction like bruxing or nail biting, cementum lesions related to eruption collision, cracks or invagination grooves, periodontal pathology and surgery, trauma and even viral infections.