Autistic Spectrum Disorder (ASD) is characterised by difficulties with social interactions, communication and rigid I stereotyped behaviours, with a prevalence of around 1 % within the population. Research has shown that children with ASD also have heightened feelings of anxiety compared to typically developing peers, particularity with social anxiety. Cognitive Behavioural Therapy (CBT) has empirical evidence that demonstrates its efficacy in supporting children with ASD to manage their anxiety. However, these studies have only shown improvements in the children's anxiety using standardised questionnaires. As such, it is difficult to infer whether the gains made using CBT are long-term, or whether it leads to a qualitative improvement in children's interactions with their community. Typically, CBT is typically delivered by Child and Adolescent Mental Health Services, which can be inaccessible to some children and their families. This study employed a mixed methods approach to understand the effectiveness of a six week, group administered, secondary schoolbased CBT programme. 28 children took part in the research, with 14 in the treatment-as-usual group and 14 in the experimental group. All children completed the Wechsler Abbreviated Scale of Intelligence, the Social Responsiveness Scale, Spence Children's Anxiety Scale - parent and child versions (SCAS-P/C), the Coping Scale for Children and Youth (CSCY). Qualitative data was also collected through parent and child interviews using a semi-structured technique. Postintervention data consisted of the SCAS-P/C and the CSCY and further parent and child interviews. Follow-up measures were taken six to eight weeks after post-intervention using the SCAS-Parent and child versions and the CSCY. Results suggest children who took part in the intervention had reduced levels of anxiety compared to the TaU group, both at post - intervention and follow-up. However, these improvements were not at a clinically significant level. Interview data, analysed using Thematic Analysis, provided unique insight into the process of cognitive change, the nature of anxiety in children with ASD and highlighted potential barriers to change for these children. Furthermore, the parents identified a lack of post diagnostic support and the view of their child's constantly changing profile of needs. The results are related to their implications for the professional educational psychologist, who is considered to be well placed to respond to the identified needs of this group and to implement CBT programmes in schools. Methodological issues and weaknesses are discussed as well as implications for further study.