Upper airway diseases decrease sleep time and/or quality, which leads to excessive day time sleepiness, fatigue, and lack of concentration. Upper airway diseases can be classified into two groups: upper airway resistance syndrome and sleep apnea syndrome. Sleep apnea syndrome is a disease, which occurs during sleeping, characterized by apneic and hypopneic events, low oxygen levels in lungs, blood oxygen desaturation, sleep arousal, and/or awakening. It increases morbidity and mortality of the patients. Sleep apnea syndrome has three types: central, mixed, and obstructive. Central and mixed apnea can be treated only by medical practitioners. But in the treatment of obstructive sleep apnea, orthodontists and maxillofacial surgeons have an important role. The aim of this chapter is to present current information about etiology, diagnostic tools, and treatment alternatives of obstructive sleep apnea and to introduce dental sleep medicine to orthodontists.