Publisher Summary The chapter deals with the various types of dementias, differential diagnosis, diagnostic criteria, guidelines for assessment, and common measures to assist in the diagnosis of dementia in individuals with intellectual disabilities (ID). Memory disorders received increased attention over the last few decades, leading to a number of advances and refinements in their assessment and identification. Individuals who have developed the onset of dementia are now provided with better care at an earlier stage in their disease than in the past. These individuals have greater opportunities to seek and receive care that may slow the progression of their disease, as well as greater opportunities to maintain their independence and quality of life.