The active form vitamin D is a seco-steroid with multiple neurotrophic and neuroprotective functions in the central nervous system. Robust evidence from studies in animals suggests that vitamin D deficiency may impair brain physiological functioning causing anatomical and behavioral adverse effects. On the other hand, vitamin D has been found to be protective against biological processes associated with Alzheimer's disease and cognition, including amyloid-β deposition, inflammation, calcium homeostasis, and corticosteroid-induced perturbations in cortical areas and the hippocampus. Human studies that examined the relationship between vitamin D status and cognitive function have provided inconclusive results. The majority of cross-sectional and longitudinal studies suggest a potentially protective association, whereas results from clinical trials are mostly negative, or at best, controversial. We review these studies in humans, with particular emphasis on randomized and observational prospective ones.