Pediatric obstructive sleep apnea (OSA) was initially described in 1976. In 1981, Dr. Guilleminault emphasized that pediatric OSA was different from the clinical presentation reported in adults. It was characterized by more disturbed nocturnal sleep than excessive daytime sleepiness, and presented more behavioral problems, particularly school problems, hyperactivity, nocturnal enuresis, sleep terrors, depression, insomnia, and psychiatric problems. The underlying causes of pediatric OSA are complex. Such factors as adenotonsillar hypertrophy, obesity, anatomical and neuromuscular factors, and hypotonic neuromuscular disease are also involved. Adenotonsillectomy (T&A) has been the recommended treatment for pediatric OSA, but in the recent past this practice has been placed very much in question. Therefore, we will discuss the mechanism of pediatric OSA and investigate obese and nonobese pediatric sleep-disordered breathing. Moreover, the important concept that dysfunction leads to the dysmorphism that impacts on the size of the Lin H-C (ed): Sleep-Related Breathing Disorders.