**Abstract**

Vitamin D is a hot topic that has attracted attention over the past 10 years, especially since a large proportion of people suffer from this nutrient deficiency. Vitamin D deficiency is estimated to be about 1 billion people all over the world and 50% in Asia and the Middle East. Saudi Arabia has also demonstrated a high prevalence of vitamin D deficiency among healthy Saudi individuals. This chapter provides, in detail, a clear and understandable identification of vitamin D, its function, source, synthesis, metabolism, status, and deficiency. The chapter also focuses on studying vitamin D deficiency in Saudi Arabia based on PubMed's initial research criteria.

**Keywords:** vitamin D, function, synthesis, deficiency, Saudi Arabia
