Background and Aims: Screening and assessment of cognitive changes in adults with Intellectual Disabilities, mainly Down Syndrome (DS), is crucial to offer appropriate services to their needs. We present a systematic review of the existing instruments assessing dementia, aiming to support researchers and clinicians’ best practice. Methods: Searches were carried out in the databases Web of Science; PubMed; PsycINFO in March 2019 and updated in May 2020. Studies were selected and examined if they: (1) focused on assessing age-related cognitive changes in person with ID; (2) included adults and/or older adults; (3) included scales and batteries for cognitive assessment. Results: Forty-eight cross-sectional studies and twenty-six longitudinal studies were selected representing a total sample of 5,851 participants (4,089 DS and 1,801 with other ID). In those studies, we found 38 scales, questionnaires, and inventories, and 14 batteries for assessing cognitive and behavioural changes in adults with DS and other ID. Conclusion: The most used instrument completed by an informant or carer was the Dementia Questionnaire for Learning Disabilities (DLD), and its previous versions. We discuss the strengths and limitations of the instruments and outline recommendations for future use.