Prader-Willi syndrome (PWS) is a rare, genetic, multisystemic disorder, characterised by short stature, muscular hypotonia, intellectual disability, behavioural and psychological problems and frequently hypogonadism and impaired growth hormone secretion. PWS arises due to loss of function of paternally-expressed, imprinted genes from chromosome region 15q11-q13. In new-borns and infants, the syndrome presents symptoms of muscular hypotonia and nutritional difficulties, which from the age of approximately two years is replaced by hyperphagia often leading to severe obesity. With increasing age the behavioural and psychological problems increase while the muscular hypotonia improves. During the last decades the knowledge of different aspects of PWS has increased and accordingly the treatment possibilities. However, no specific treatment exists and thus problems continue lifelong. Taking care of PWS patients is therefore a demanding task and necessitates a solid knowledge and understanding of the disorder as well as a multidisciplinary approach. This book will provide professionals taking care of patients with PWS with updated and comprehensive information and advise. The content is built up logically and it will be easy to navigate between the chapters, which cover all clinically relevant issues, genetics, signs and symptoms in children and adults, treatment, words from patients and parents, supporting organisations, and from a parent to a PWS patient.