Obstructive sleep apnea syndrome is the most common organic sleep disorder resulting in excessive daytime somnolence. It is almost as common as asthma. According to recent epidemiologic studies, the prevalence of obstructive sleep apnea syndrome is probably about 2% in women and somewhere around 4% in adult men in general. Many elderly people have the syndrome, and it is very common among patients who are morbidly obese, acromegalic, asthmatic; patients with arterial hypertension and heart disease, those with adult onset diabetes; and among patients with craniofacial abnormalities. In those groups, more than 30% or 40% of patients may have obstructive sleep apnea syndrome. Even more patients may have sleep apnea without daytime symptoms or partial upper airway obstruction during sleep. Among children, symptoms such as snoring and apneic episodes are relatively rare, but a high proportion of children with these symptoms have hypoxic respiratory events. Some recent methodologic issues and use of questionnaires are discussed.