Objective To provide an overview on "orthodontic therapy in periodontitis patients" by addressing the following three questions: (1) At which time point can orthodontic treatment start in periodontitis patients? (2) What should be considered during orthodontic treatment? (3) Can teeth with a reduced periodontium be maintained after orthodontic treatment? Results In general, the scientific evidence on this topic is mainly based on preclinical, retrospective studies, and case series; controlled clinical trials are scarce. Nevertheless, it is clear, that orthodontic tooth movement should be performed only in periodontally healthy (i.e., non-inflamed) tissues, otherwise further attachment loss might occur. Therefore, cause-related periodontal therapy should always precede orthodontic treatment and supportive periodontal treatment should be continuously provided during orthodontic therapy. Yet, up-to-now, there is no clear treatment recommendation regarding the time-point and type of any surgical intervention before orthodontic treatment. In relation to treatment of patients with vertical defects it is, however, not clear whether an open flap debridement is sufficient or application of any additional regenerative material will improve the prognosis. The results of orthodontic treatment can be successfully preserved on the long-term – also on a reduced periodontium – if sufficient stabilization and periodontal maintenance is provided. Conclusion Orthodontic therapy has no negative influence on teeth with a reduced but healthy periodontium and the results can be maintained for a large period of time. Hence, cause-related periodontal therapy should always precede orthodontic treatment to establish inflammation-free conditions, but up-to-now there is no clear recommendation regarding the time-point and type of periodontal surgical interventions.