There are now ample guidelines for the assessment and diagnosis of possible dementia in individuals with intellectual disabilities (ID) and Down syndrome. However, little is known about their implementation in clinical practice. This study set out to examine the clinical practice of one key professional group, namely clinical psychologists. A national survey of clinical psychologists in ID services in the United Kingdom was undertaken. Detailed descriptions of clinical practice were obtained from 64 psychologists. Responses were further explored in focus groups. The results suggest marked variability in practice, assessment methods, and explanations clinicians give to service users and carers. Clinicians described struggling with the ethics and practicalities of how to present dementia assessments to individuals with ID and highlighted a need for more research, debate, and guidance. They also noted numerous shortfalls in service provision once aging individuals with ID show signs of dementia. Further research and clearer consensus guidelines are needed to improve assessment and diagnosis for this group.