This chapter covers medically unexplained symptoms (MUS) and functional disorders. More than half of the patients observed in primary care or in population-based studies are medically unexplained, so persons manifesting some kind of symptoms, whether they seek treatment or not, may or may not suffer from some identifiable disease, although ICD-10 and DSM-5 are listing a lot of symptoms as if they were diseases. The symptom iceberg represents the visible fraction observed by physicians. Actually, 26–33 per cent of patients seen in a practice of internal medicine betray unexplained medical disorders. If one excludes respiratory infections and skin disorders, the incidence of unexplained conditions rises to more than 50–70 per cent. If the concept of disease dominates our medical thinking, functional disorders, by contrast, make the lion’s share of medical care as they make up the bulk of reported and unreported medical complaints, and are responsible for a large amount of loss of absenteeism, productivity, and medical expenses. Functional disorders make the lion’s share of medical care and are responsible for a large amount of absenteeism, loss of productivity and medical expenses. The predicate diseased does not universally quantify the predicate sick.