Vitamin D is now considered as an important player in the development and function of the nervous system as well as a therapeutic tool in a number of neurological pathologies. Numerous experimental and preclinical data suggest a link between vitamin D status and cognitive function. Human studies confirm the existence of a correlation between low levels of circulating 25-hydroxyvitamin D (25(OH)D) and cognitive impairment or dementia in ageing populations. In parallel, animal studies show that vitamin D supplementation protects against the biological processes associated with Alzheimer's disease and improves learning and memory performances in various animal models of ageing and Alzheimer's disease. However, interventional clinical studies are disappointing. Only one in five randomized controlled trials reported a significant improvement in cognition and one study showed an enhancement in the Mini-Mental State Examination score, when vitamin D was administered with memantine. To better assess the therapeutic potential of vitamin D in the ageing brain, future studies should carefully consider the possible implications of genetic heterogeneity, sex- and age-related responses, individualized supplementation according to baseline, the time window for administration.