Vitamin D plays important roles in regulating calcium metabolism and bone metabolism. It maintains appropriate serum calcium concentrations by promoting active calcium transport in the small intestine and calcium reabsorption in the kidneys, and controls the balance between bone resorption and formation. Rickets and osteomalacia are typical vitamin D deficiencies (severe deficiency), both characterized by notable bone abnormalities. On the other hand, it has been reported that a decrease in vitamin D status (insufficiency or deficiency) that does not result in overt deficiency symptoms is involved not only in bone density and fractures, but also in falls, cardiovascular and immune system disease risk, and COVID-19 severity. This chapter provides an overview of the fundamentals of vitamin D, its role in health, and the current vitamin D status.