The purpose of this thesis was to determine whether there is a specific behavioural phenotype associated with Rett syndrome (RS). In the first of two studies, behavioural and emotional features were assessed in girls with RS and girls with severe to profound learning disabilities (SLD). A well- validated checklist was used to assess general behaviour disorder and the Rett Syndrome Questionnaire (RSQ) was developed to measure RS-specific behaviours. Despite having a similar level of overall behavioural disorder, the girls with RS showed a specific profile of behavioural disturbance characterised by a greater number of autistic relating behaviours and less antisocial behaviours. Results from the RSQ revealed there to be eight clusters of RS-specific behaviour. Of these, hand behaviours, breathing problems, and difficulties in walking are currently included in the diagnostic criteria for RS. The findings suggest that night-time behaviours, fear/anxiety, body rocking and expressionless face behaviours, general mood, and repetitive face movements might also help to delineate girls with RS from other girls with SLD. Age, level of physical ability, epilepsy, and early developmental and medical difficulties could only explain a small proportion of the variability in both general behaviour disorder and RS-specific behaviour within the RS groups suggesting there are other, biological or environmental, factors involved. The profile of RS-specific behaviours in the girls with RS was compared to that in a sample of adult females with RS, although few strong developmental trends were apparent. In the second study, autistic behaviours were assessed in more detail in a sub-sample of girls with RS and SLD. Girls with RS were found to have a higher number of autistic behaviours than expected based on their developmental level. Findings from both studies provide evidence for a specific profile of behaviour associated with RS.