Vitamin D3, or cholecalciferol, is the naturally occurring form of vitamin D that is converted in the skin and hydroxylated in the liver and kidney to the active form found in humans. The main role for vitamin D is calcium homeostasis, and low levels of vitamin D result in lower gastrointestinal absorption of calcium. Vitamin D is also critical for mineralization of bone tissue, muscle function, and coordination. Recent studies have found prevention of bone mass loss and reduction in falls and fractures in patients supplemented with vitamin D. A high percentage of systemic lupus erythematosus patients are reported to have insufficient or deficient levels of vitamin D. This paper reviews the biology of vitamin D, its role in calcium homeostasis, and how it contributes to the maintenance of bone, muscle, and joint function in older adults and individuals with systemic lupus erythematosus.