Background: Views can be collected from individuals (self-report) or others on their behalf (proxy-report). Objective: We aimed to review the literature on methods and statistical approaches used to evaluate observer versus self-report responses from individuals with intellectual disability or Down syndrome. Methods: A series of key questions related to statistical approaches and data collection methods were formulated a priori to inform the search strategy and review process. These addressed the topics of self-report in individuals with intellectual disability, including Down syndrome. Using the National Library of Medicine database, PubMed, detailed literature searches were performed. The quality of available evidence was then evaluated, the existing literature was summarised, and knowledge gaps and research needs were identified. Results: Fifty relevant original articles were identified which addressed at least one key question. Study details, including: research design, internal validity, external validity, and relevant results are presented. Review of studies of individuals with intellectual disability which used a variety of statistical approaches showed mixed agreement between self-report and proxy-report. Conclusion: Few studies identified to-date have used self-report from individuals with Down syndrome, but lessons from the existing intellectual disability literature can guide researchers to incorporate self-report from individuals with Down syndrome in the future.