Dementia is a major challenge in contemporary intellectual disability care. People with Down syndrome have an exceptionally high genetic risk to develop Alzheimer’s disease: 50-80% develops the disease. Remarkably, whereas dementia symptoms present early in the one person, the other remains free of symptoms until late in life. Who will develop dementia, and who will not (yet)? Predicting the disease progression is particularly difficult in this population. Comprehensive evaluation of behavioural and psychological symptoms of dementia may aid. Besides cognitive decline, dementia is characterized by behavioural changes. Studies in the general population have shown that specific changes already present before the diagnosis is established. As such, behavioural changes may serve as early alarm signals for (approaching) dementia. Surprisingly, these changes have hardly been studied in people with Down syndrome. This dissertation focuses on behavioural changes and underlying alterations in neurotransmitters in the brain. Next to biological results, this dissertation presents the important findings in a unique, large European behavioural study in 281 people with Down syndrome with and without dementia. Using the newly developed BPSD-DS scale, it was shown that the frequency of anxiety, sleep problems, agitation, aggression, apathy, depressive symptoms and eating/drinking problems evidently increases in the group with dementia. Among those with questionable dementia (a kind of prodromal stage), a substantial number already showed an increase in anxiety, sleep problems, apathy and depressive symptoms – possible early alarm signals. Early identification is of utmost importance for adaptation of daily care and to enable therapeutic interventions to increase the quality of life.