For several decades, orthodontists and periodontists have repeated that orthodontics have no harmful consequences on the periodontium when it is healthy or treated. However, a systematic review of the literature undermines this common refrain and concludes that there is a lack of reliable evidences of the positive effects of orthodontics on periodontal health with, in the best periodontal conditions, slight adverse effects. It is therefore the responsibility of orthodontists to keep the periodontal cost of orthodontic treatment as low as possible. How to make sure that this « at best » does not turn into an « at worst » ? In order to minimize the deleterious consequences of orthodontic treatment on the periodontium, the orthodontist must be able to specify which patients she/he can immediately consider providing orthodontic treatment and those on whom periodontal treatment is mandatory before all. The orthodontist must therefore transform, for a few minutes, into a periodontist in order to recognize the eight signs of loss of attachment and the six risk factors for periodontitis, exposed in this article. Both needs for periodontal and orthodontic treatment have to be measured and would not be efficient without the patient's and the practitioners' motivation.