The study was designed to (i) investigate the behavioural characteristics of individuals with Rett syndrome and (ii) the impact that severity of behavioural and clinical symptoms has on family well-being. A cross-sectional study was conducted to assess differences in behavioural characteristics within subjects and across groups with rare genetic syndromes using standardised measurements. The sample was followed-up after 16 months to examine developmental changes. Three studies were conducted: a national survey of individuals with Rett syndrome and their families, direct behavioural observation of 11 individuals with Rett syndrome in the natural environment and a longitudinal follow-up of the national survey. Results confirmed that behaviours such as hand stereotypies, breathing abnormalities and sleep disturbances are typical of the syndrome. Other behaviours, such as autistic features, impulsivity and overactivity, self-injurious behaviour and depression were also investigated. Although the behaviours were reported in some of the participants, these were not typical of the syndrome. Although some trends were highlighted in the analysis of the longitudinal study, the behavioural features of the group were found to be stable over time. Family stress, anxiety and depression were found to be related to increased severity in areas such mood, fear/anxiety, body rocking and expressionless face and not related to the severity of the clinical phenotype. Results of the longitudinal family study were consistent with the cross-sectional study in that increased severity of behavioural problems is linked to worse maternal psychological well-being. Behaviours mostly seen in the group who were observed were: hand stereotypies, breathing abnormalities and self-injurious behaviours. Hand stereotypies were very frequent for the vast majority of the participants. However, analysis suggested that these stereotypies were less frequent when the girls/women were engaged in another activity using the hands. Breathing abnormalities were observed in the younger girls and the behaviour tended to attract adult attention.