Background and Aims: Despite a growing interest in the use of Cognitive Behavioural Therapy with individuals with Autism Spectrum Disorders, there has been little systematic appraisal of effectiveness research in this area to date. The primary aim of the current study was to systematically appraise the evidence for using CBT in the treatment of either core features of ASD or cooccurring mental disorder in individuals with ASD across the lifespan. Methods: A systematic search of relevant databases was conducted according to pre-defined criteria, followed by a series of random effects meta-analyses to account for the variation in outcome report type. Results: Fifty studies met inclusion criteria and 48 studies, involving 2099 participants (1081 CBT, 1018 control) were included in the meta-analysis. CBT for the treatment of mental disorder was associated with a significant “medium” effect size, g = .66, for informant-reported measures, and a significant “medium” effect size, g = .73, for clinician-reported measures. Similarly, CBT for the treatment of core features of ASD was associated with a significant “small” effect size, g = .48, for informant-reported measures, a significant “medium” effect size, g = .65, for clinician-reported measures, and a significant “small” effect size, g = .35, for task-based measures. CBT was not found to be superior to control when self-reported outcome measures were utilised. Sensitivity analyses to exclude outliers and studies deemed to be at a high risk of bias generally reduced effect size magnitude. Subgroup analysis was severely limited by a lack of definitive studies and the interpretation of results was hampered by the poor methodological quality of included studies. Conclusions: Future larger-scale clinical trials are needed to further explore the effectiveness of CBT in this client group, with well characterised samples, clearly defined primary outcome measures and adequate randomisation, allocation concealment and blinding.