Prader Willi Syndrome (PWS) is the most common genetic cause of obesity with incidence of 1: 15,000 to 1:25,000 live births. It is the first human syndrome identified as due to a genomic imprinting defect and most cases occur sporadically. PWS is characterised by hypothalamic dysfunction with hypogonadism and growth hormone deficiency occurring almost universally. Thyrotropin and ACTH deficiency may also occur. Decreased intellectual function is a feature, with IQ usually borderline to mildly slow, as well as centrally driven maladaptive behaviours with the hallmark being hyperphagia. Initially infants present with failure to thrive, poor muscle tone and poor suck but by age 2-4 yr their appetite and food intake markedly increase resulting in obesity. Early diagnosis and interventions to prevent obesity and the associated complications are critical. We run a specialised PWS clinic with 40 adults under our care at present. One third of our cohort has type 2 diabetes mellitus. Management issues in this unique patient group will be discussed.