Many individuals on the autism spectrum experience sleep difficulties. This includes sleep onset delay, night wakings, early morning wakings and a lower sleep efficiency. Sleep difficulties are associated with a number of detrimental effects on the daytime functioning of people on the autism spectrum. Research for sleep interventions for adolescents and young adults on the autism spectrum (henceforth referred to as young people) is limited with very few studies evaluating cognitive and behavioral treatments in this population, or young- person-led and implemented sleep interventions. Cognitive behavioral therapy for insomnia (CBT-I) is considered the first line treatment in the treatment of insomnia among typically developing adults, however research regarding its effectiveness in young people on the autism spectrum is currently limited. Additionally, many sleep interventions for those on the autism spectrum focus on parent-implemented interventions, leading to difficulties in accurate reporting and the developmentally appropriate approaches. This thesis evaluated the effectiveness of a CBT-I programme for sleep problems in three young people on the autism spectrum. Treatment consisted of a minimum of eight, weekly sessions that were approximately one hour in length, delivered in-person or via Zoom. Components of the CBT- I programme were individualised to suit the support requirements and interests of the young person. All young people experienced difficulties with sleep onset latency (SOL), and two young people had difficulties with night wakings (NW). All participants completed the CBT- I programme and two participants provided post-treatment data, including four- and 14-week follow up data. The collateral benefits of improved daytime functioning and behavior on young people were assessed using the Gilliam Autism Rating Scale (GARS-3), Multidimensional Anxiety Scale for Children (MASC-2), Child Behaviour Checklist (CBCL), Youth Self Report Form (YSR), Strengths and Difficulties Questionnaire (SDQ), and the Pediatric Quality of Life Index (PedsQL), which were administered pre- and post-participation in the programme. In response to the CBT-I programme, all participants demonstrated a decrease in their SOL, and NWs were decreased for the two participants for whom this was an intervention target. Results from the Adolescent Sleep Hygiene Scale (ASHS) and Adolescent Sleep Wake Scale Revised (ASWS-R) demonstrated improvements in sleep hygiene practices, perceived sleep, and ability to fall asleep. Results of GARS-3 and MASC-2 suggested improvements in Autism Index scores and anxiety symptoms following treatment. Mixed results for other internalising and externalising behaviors were found on the CBCL, YSR, SDQ and PedsQL. Overall, the findings demonstrate the effectiveness of a young-person-led CBT-I programme for addressing problems in young people on the autism spectrum. These findings have a number of important implications for clinical practice, including for adaption of treatments for those on the autism spectrum, and highlight the importance of including young people on the autism spectrum in the assessment, design, and intervention process. Research findings also highlight the need for further research into the effects of a CBT-I programme on a larger age range, sample size and gender distribution.