Aim: Oral dysfunction, including thumbsucking, mouth breathing, and infantile swallowing, leads to abnormal growth and development of craniofacial structures. This work is designed to assist in the diagnosis and management of oral dysfunction as they pertain to orthodontic treatment and outcome. Methods: The review of patient records and the literature review indicate that successful elimination of oral dysfunction improves orthodontic treatment results. A variety of oral habits, both plastic and relatively aplastic, are discussed, along with their roles in the development of craniofacial abnormalities and their treatment alternatives. Conclusion: Oral dysfunction may cause changes in the craniofacial skeleton and may affect craniofacial muscular function. However, to fulfill the demands of evidence-based science, further study is needed on this probable cause-effect relationship. World J Orthod 2000;1:125-133.