There are several situations in clinical orthodontics when the crown-root ratio of single teeth in treated cases may become questionable according to some prevailing concepts. As far as I know, one should aim at a minimum 1:1 crown-root ratio for teeth, if they are to be successfully retained in a long-term perspective. However, sometimes this optimal crown-root ratio may not be achieved. For example, it may exceed a 1:1 proportion on single teeth in some adult cases with advanced periodontal disease with pronounced alveolar bone loss; after forced extrusion of roots associated with traumatic injuries, crown-root fractures, endodontic perforations, and for other reasons; as a consequence of marked apical root resorption on maxillary incisors during the therapy, etc. My questions in this regard deal with currently accepted concepts regarding increased mobility and survival of teeth with poor crown-root ratio, such as: (1) What are the evidence-based concepts regarding increased tooth mobility? (2) What is pathologic tooth mobility, and how should it be treated? and (3) Is it, in fact, necessary in orthodontics to end with a 1:1 crown-root ratio for all teeth after treatment? —Jack Sheridan, New Orleans, USA