Understanding the functional aspects of the orofacial region is a prerequisite for arriving at a proper diagnosis and planning treatment for orthodontic patients. The morphology of the skeleton and the arrangement of the teeth are mainly determined by facial growth and the development of the dentition, over which functional aspects have a large influence. The mode of breathing; the position of the tongue, lips, and cheeks; the interdigitation of the posterior teeth; and the forces provided by the occlusion all contribute to the developmental process. Functional aspects affect orthodontic therapies, influence the result that can be achieved, and determine to a large extent the changes that occur in the dentition when retention devices are discarded. Produced in six languages (English, Spanish, French, German, Italian, Portuguese).