Rett syndrome is a disorder occurring in girls who, prior to the onset of the disorder (usually between six and eighteen months) have appeared to develop normally.

Rett syndrome is characterised by a four-stage clinical course of development involving sequential progression. As the child progresses through the stages the characteristics change - with each stage having its own defined characteristics, along with an onset of new behaviours or symptoms and the reduction of others.

The disabling and handicapping effects of the syndrome are multifaceted with symptoms of the syndrome impacting on cognitive as well as physical development. Rett syndrome has been described as leaving children "profoundly mentally and physically disabled".