The varying extent of permanent tooth resorption attributable to different types of orthodontic appliances is a continuing cause for concern. This paper reviews recent researches which contribute to an improved understanding of the origin and behaviour of osteoclasts; and considers the structure and behaviour of the periodontal ligament and its relationship to the cementum and bundle bone of the tooth socket. Permanent tooth resorption is identified as a normal phenomenon. A simple explanation is offered as to why under normal circumstances the bone of the tooth socket is more likely to be resorbed than the tooth itself. With this knowledge treatment procedures can be selected which will reduce tooth resorption risks to a minimum.