The major and most well-known function of vitamin D is to maintain calcium and phosphorus homeostasis and promote bone mineralization. However, recent evidence suggests that vitamin D may be important for a variety of nonskeletal outcomes. The review synthesizes the available evidence for the role of vitamin D in skeletal health as well as its novel roles in medical conditions such as muscle function, falls, immunity, glucose homeostasis, and cardiovascular diseases. The article reviews methods for assessing vitamin D status and suggests strategies to restore vitamin D status in patients requiring enteral or parenteral nutrition who are at particularly high risk of hypovitaminosis D. Screening for hypovitaminosis D with plasma total 25-hydroxyvitamin D should be a routine part of the care of the patient requiring enteral or parenteral nutrition. Restoration of optimal vitamin D status with high-dose supplemental vitamin D is required in most cases, whereas exposure to sunlight or an ultraviolet B radiation-emitting device is most effective in patients with severe malabsorption or those requiring long-term parenteral therapy. Given the emerging role of vitamin D for a variety of acute and chronic conditions, the optimal vitamin D status in acutely ill patients as well as in patients requiring long-term nutrition therapy warrants further investigation.