Obstructive sleep apnea (OSA) is a sleep-related breathing disorder which is characterized by intermittent hypoxia and frequent arousals because of partial or complete upper airway obstruction during sleep. This condition may be associated with various clinical sequelae such as hypertension, cardiovascular disorders, and metabolic syndrome in adults, and failure to thrive, deformity of dentofacial structures, behavioral problems, impairment of sleep-related quality of life, and enuresis in children. Although technician-attended, in-laboratory, overnight standard polysomnography (PSG) is considered a basic and important diagnostic test to classify the severity of respiratory disturbance and define the proper treatment modality in adults and children with OSA, it is too expensive and complicated to perform. Recently, in order to overcome the disadvantages of standard PSG in the clinical field, a portable sleep monitoring device has been introduced as an alternative to standard PSG in selected patients. The purpose of this article is to elucidate the clinical indications of polysomnography in OSA, to define the differential characteristics of polysomnographic indices between children and adults with OSA, and to introduce the clinical efficacy of a portable sleep monitoring device.