Older persons with dementia may become confronted with a decline in the level of physical activity. Indeed, a positive relationship between physical activity and cognition has been demonstrated. Although the causality of this relationship needs to be confirmed in advanced dementia, particularly animal experimental studies show the possible negative influence of restrained physical activity on behavior of patients with dementia. Patients with dementia, who get immobilized because of agitation and restlessness, may show an increase in these two symptoms. Another cause for reduced physical activity or inactivity may be the experience of pain. Pain experience may even increase in dementia by neuropathological changes in the central nervous system. There is an increasing (inter)national interest for the development of a more reliable assessment and treatment of pain, for the causality of the relationship between pain and physical (in)activity, and for the causality of the relationship between physical (in)activity and cognition in dementia. In the present paper, the various topics will be addressed in this order.