This article gives an overview of the behavioural problems and genetic abnormalities of Prader-Willi Syndrome (PWS). PWS is associated with an absence of a paternal contribution to a defined area of chromosome 15. The syndrome is characterised by extreme hyperphagia which starts in early childhood. Most adults with PWS are overweight resulting in a potentially reduced life expectancy. Certain behavioural and personality characteristics appear to be more common in PWS than in other people who have similar overall cognitive impairments. Psychiatric abnormalties such as psychosis have been reported in people with PWS. The study of patients with PWS offers an opportunity to relate specific behaviours to defined genetic abnormalities.