Only a few studies exist on cognitive development across the life span of people with Down’s Syndrome (Trisomy 21). The main objective of this study is to investigate the cognitive functioning from childhood to older adult age and to examine differences to age-matched, typically developing controls. For this purpose we use neuropsychological tests, questionnaires concerning psychiatric disorders and symptoms of dementia, and magnetic resonance imaging (MRI). Findings on developmental trajectories will expand the state of research on cognitive functioning in Down’s Syndrome, improve neuropsychological assessment in patients with an intellectual disability and provide a basis for future clinical trials.