The aim of this study was to systematically investigate the literature for articles referring exclusively to the duration of orthodontic therapy and to explore the various factors that could affect this. A Medline search from 1990 to the first week of March 2005 was conducted, the Cochrane Database for Systematic Reviews was utilized, five orthodontic journals were hand searched, the abstracts of related articles were reviewed to search for any relevant studies that might have been missed, and the reference lists of the retrieved articles were hand assessed. Eligibility was determined by reading the reports identified by the search. The end result of this search provided 41 articles. Although there is a need for more conclusive research, the present review revealed several conclusions concerning the duration of orthodontic treatment: (1) there are indications that extraction treatment lasts longer than the non-extraction therapy; (2) age does not seem to play a role provided the patients are in the permanent dentition; (3) when Class II division 1 malocclusions are considered, there is evidence that the earlier the orthodontic treatment begins the longer its duration; (4) there is conflicting information regarding treatment duration within public health systems; (5) combined orthodontic-surgical treatment duration is variable and appears to be operator sensitive; (6) various factors, such as the technique employed, the skill and number of operators involved, the compliance of the patients, and the severity of the initial malocclusion, all seem to play a role; and (7) impacted maxillary canines appear to prolong treatment.