Prader-Willi syndrome is a rare genetic disorder, affecting 1 out of 25,000 births, in which a critical region of chromosome 15, the 15q11-q13 region, is affected. At birth, PWS infants exhibit severe hypotonia that partially improves, explaining in part suckling and swallowing troubles and the delay in psychomotor development. Characteristic facial features (dysmorphic syndrome) and very small hands and feet are frequently observed at this age. After this initial phase, the most striking signs appear: hyperphagia and absence of satiety often leading to severe obesity in affected children as young as two years. The situation may deteriorate quickly without adequate outside controls and explains in great part the morbidity and mortality of these patients. Other endocrine abnormalities in association with the hypothalamic-pituitary abnormalities contribute to the clinical picture of short stature due to a growth hormone deficiency and incomplete pubertal development. The degree of cognitive dysfunction varies widely from one child to another. It is associated with learning disabilities and impaired speech and language development worsened by psychological and behavioural troubles. The expert consensus is that diagnosis should be based on clinical criteria (Holm's criteria of 1993, revised in 2001) with confirmation by genetic study. Most cases are sporadic and familial cases are rare, those informations should be given as genetic counselling. It is necessary to set up a global and multidisciplinary management. Early diagnosis, early multidisciplinary care and growth hormone treatment have greatly improved the quality of life of these children. We have no long-term data on the effect of GH treatment in adults, on behavioural troubles and autonomy of the persons. In adults, complications particularly linked to obesity and problems of autonomy are still very important.