Mouth breathing often results from the narrowing and obstruction of the upper airway. There are inconsistent opinion about the causal effect between month breathing and the specific craniofacial morphology. The purpose of this article is to review some of the literature related to the structure of the upper airway, the etiology of mouth breathing, and the possible cause-effect relationships between respiratory mode and craniofacial characteristics. Orthodontists are well positioned to treat the patients who suffer from less threatening forms of obstructive sleep apnea/hypopnea syndrome. This article also reviews the information about this syndrome.