Vitamin D is a secosteroid that plays an important role in the central nervous system (CNS). Through binding to the vitamin D receptor (VDR), which is found throughout the CNS, 1,25-hydroxyvitamin D
regulates gene transcription to exert neurotrophic and immunomodulatory effects. As a result of various downstream responses, vitamin D signaling provides neuroprotection, decreasing damage and accelerating recovery from a variety of CNS insults. As a result, a growing body of scientific literature addresses a possible relationship between this steroid, or lack thereof, and a variety of neurodegenerative diseases. In the first sections of this chapter, we will summarize the current understanding of the role vitamin D plays in CNS protection and development. In the latter section, we will examine existing evidence that vitamin D plays a role in neurodegenerative diseases, specifically evaluating if vitamin D availability may modify the risk, prognosis, and treatment outcomes for patients with Parkinson disease, Alzheimer disease, amyotrophic lateral sclerosis, and multiple sclerosis.