Since it was shown that numerous neurological, psychiatric and internal illnesses have characteristics manifested during, or influenced by sleep, somnology has been playing a clinically more and more important role. Among the 88 diagnoses listed by ICD, not only insomnia, but also sleep-related respiratory disorders, in particular the obstructive sleep apnea syndrome, are of special importance. Sleep apnea is associated with coronary heart disease, myocardial insufficiency and other pathological conditions. Already in the doctor's office, a carefully taken history (nocturnal apnea alternating with irregular snoring, and diurnal sleepiness) can arouse an appropriate suspicion. This can be confirmed by an ambulatory polygraphic exploration. The definitive diagnosis is then established with the aid of polysomnography in the sleep lab where specific treatment is also initiated.