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A systematic review and meta-analysis was conducted to explore the Cortisol Awakening Response (CAR) in children with Autism Spectrum Disorder (ASD), in comparison to their typically developing (TD) peers. The CAR, characterised by an increase in cortisol in the 30-60 minutes after waking, is widely becoming used as a physiological marker of stress in the general population. Given the prevalence of anxiety and stress reported in people with ASD, this review aimed to explore whether children with ASD show the expected pattern of CAR seen in the general population, and therefore, whether this would be a useful way of measuring stress in this population. Results from the meta-analysis indicated no significant differences in CAR demonstrated by children with or without ASD. However, the review highlighted limitations in the current research; a lack of adherence to recommended CAR sampling protocol, few studies with a matched control group, and poor control of potentially confounding variables. The review highlighted wide variability in results shown by ASD groups across studies, indicating the need for further exploration of the CAR in this population to be confident of its use as a reliable marker of physiological stress in this group. The subsequent empirical paper extended the current literature regarding the CAR in children with ASD, and contributes to the understanding of the experience of anxiety in this population. Using a mixed methods design incorporating questionnaires, interviews, physiological data (cortisol) and experience sampling data, the study explored the experience of anxiety and CAR compared to a matched control group. Results indicate a significantly smaller CAR in ASD group compared to TD, but overall comparable daily levels of cortisol (AUC). Results also show discrepancies between informants for both groups. ASD adolescents self-reported significantly fewer symptoms of anxiety than their TD peers, whilst parents and teachers of ASD adolescents reported significantly more symptoms in this group compared with the TD group. Self-reported day-to-day anxiety was also lower in the ASD group. TD adolescents self-reported higher attentional control than ASD peers. Experience sampling data indicated no significant association between cortisol levels and rated emotions at each time point. For both groups, experience sampling data indicated higher cortisol levels were associated with lower noise, unstructured activity and large group sizes. The study highlights the need for further, controlled exploration of the CAR in ASD to understand the differences in patterns shown, and further exploration of appropriate ways of assessing anxiety in this population.