This chapter briefly outlines the abnormalities underlying the Rett syndrome; to describe how the disorder may present at various stages of life; and how the associated problems may be dealt with. There are severe early disturbances of sleep rhythms in Rett syndrome suggesting early involvement of the monoamine system. The brain is immature in the Rett disorder and remains poorly equipped for the demands of maturation and ageing. The criteria for 'classic' Rett syndrome were agreed by international consensus in order to facilitate comparison between cases from different centres. Adults with Rett syndrome are shorter than the rest of their family. Gastrointestinal reflux is another common problem in adults, causing pain and loss of appetite. Poor posture, difficulties with feeding technique and gaseous distension probably contribute, and low parasympathetic tone may be implicated. Manual dyspraxia is one of the chief problems associated with Rett disorder.