Down syndrome (DS) is the most common genetic cause for Mental Retardation (MR). Many individuals with DS now are living into middle and older adulthood and experiencing age related health problems including dementia. People with Down syndrome have an increased risk of developing Dementia of Alzheimer Type (DAT) from middle age on. The comorbid picture of Down syndrome and Dementia of Alzheimer Type presents a unique challenge to the clinician in both diagnosis and management. The purpose of this article is to describe the relationship between DS and DAT and to present a framework for clinical assessment and management of dementia in people with DS.