Rett syndrome (RS) is a neurological disorder that mainly, and possibly exclusively, affects girls. Diagnosis continues to be based upon a consistent constellation of clinical features observed in all of the patients worldwide. A biological marker has not been identified. In spite of this serious limitation, it is generally agreed that RS is a distinct entity and that it is genetically determined. Although it is associated with loss of function between infancy and the fifth year of life, its course becomes relatively static thereafter, setting it apart from most of the genetic neurodegenerative disorders of childhood. Neuropathological and neurochemical studies call attention to RS as a neurodevelopmental disorder of infancy resulting in failed brain growth. Clarification of its pathogenesis may provide new insight into normal brain development.