The CAMDEX-DS-II is a fully updated second edition of a validated assessment for diagnosing dementia in people with Down Syndrome (a group known to be at particular risk of dementia), as well as others with intellectual disabilities. In order to differentiate decline due to dementia or other mental health issues from pre-existing impairment, particular emphasis has been placed on establishing change from the individual’s best level of functioning. The CAMDEX-DS-II comprises a comprehensive manual including guidance for diagnosis, a structured informant interview to collect information about presenting symptoms and clinical history, a direct participant assessment (CAMCOG-DS-II), and a picture book to use in conjunction with the participant assessment. The CAMDEX-DS-II may be used in community settings by health professionals as part of the diagnostic process, or to formalise diagnosis in the context of research. The Participant Assessment: completed directly with the person with Down Syndrome or other intellectual disabilities, and comprises a cognitive assessment (the CAMCOG-DS-II) and interviewer observations.