The dual disability literature is marked by a lack of reviews with respect to assessment of specific aetiological groups. The current review summarises the wide body of literature relevant to the assessment of psychiatric and behavioural problems in people with Down syndrome. Adequate assessment within this population requires knowledge, not only of epidemiological findings with respect to psychiatric and behavioural problems, but also an understanding of other important areas including psychometrics, neurocognitive functioning, dementia and other medical issues. These areas are reviewed, and directions for future research are proposed.