In the last few decades, the world has undergone profound demographic changes, reflected in life expectancy. We live for much longer, but we cannot say that these growing years are directly proportional to the quality of life. Dementia, in its different aetiologies, is more and more frequent with a consequent decline in the quality of life. However, cognitive decline and the onset of dementia may be delayed with the adoption of healthy lifestyle habits and therapeutic combinations that use nonpharmacological approaches such as activity and physical exercise. This chapter integrates a thorough review of the literature that characterizes ageing, cognitive decline and dementia, and it summarizes scientific evidence on the effects of physical activity on cognitive functions. Finally, recommendations are presented on the prescription of exercise for older people and elderly people with dementia. It is known that there are direct benefits (action on neurotrophic factors and neurotransmitters, among others), as well as indirect ones such as those resulting from a better vascularization of the brain. However, physical exercise requires precautions related to the conditions inherent to normal and pedagogical ageing. It is possible to perform adapted physical activity resulting from the coordination of sports and health professionals, knowledgeable of the needs and idiosyncrasies of the elderly, with cognitive decline or dementia.