Because of successful modern endodontic treatment many more teeth are retained today than formerly. However inadequate coronal restoration may reduce the length of time that such teeth remain as functional units. When root canal treated teeth must be removed the patient becomes disillusioned with the treatment programme. But restoration of endodontically treated teeth is complicated by the fact that much or all of the coronal tooth structure which normally would be used in the retention of restoration has been destroyed by caries, previous restorations, trauma and the endodontic access preparation itself. The dentist must employ the principle of substitution using posts or pins that are used to aid retention and to provide tooth support.