The article provides an updated overview of the current status of vitamin D deficiency worldwide. It highlights the increased testing and supplementation of vitamin D in recent years, but notes that large intervention studies have not shown clear benefits, possibly due to methodological limitations. Vitamin D deficiency (serum 25-hydroxyvitamin D < 50 nmol/L or 20 ng/mL) is associated with adverse skeletal outcomes and increased risk of mortality, infections, and other diseases. The optimal dose and target level of vitamin D supplementation are debated, with recommendations ranging from 400 to 2000 IU daily. The article discusses the safety profile of vitamin D supplementation, emphasizing its low cost and potential benefits in various populations, including pregnant women, children, and the elderly. It also reviews the association between vitamin D deficiency and various health outcomes, such as mortality, respiratory diseases, sepsis, organ transplantation, pregnancy complications, cancer, and diabetes. The article concludes by advocating for public health interventions to prevent severe vitamin D deficiency and suggests that vitamin D may be an important, inexpensive, and safe adjuvant therapy for many diseases.The article provides an updated overview of the current status of vitamin D deficiency worldwide. It highlights the increased testing and supplementation of vitamin D in recent years, but notes that large intervention studies have not shown clear benefits, possibly due to methodological limitations. Vitamin D deficiency (serum 25-hydroxyvitamin D < 50 nmol/L or 20 ng/mL) is associated with adverse skeletal outcomes and increased risk of mortality, infections, and other diseases. The optimal dose and target level of vitamin D supplementation are debated, with recommendations ranging from 400 to 2000 IU daily. The article discusses the safety profile of vitamin D supplementation, emphasizing its low cost and potential benefits in various populations, including pregnant women, children, and the elderly. It also reviews the association between vitamin D deficiency and various health outcomes, such as mortality, respiratory diseases, sepsis, organ transplantation, pregnancy complications, cancer, and diabetes. The article concludes by advocating for public health interventions to prevent severe vitamin D deficiency and suggests that vitamin D may be an important, inexpensive, and safe adjuvant therapy for many diseases.