Vitamin D insufficiency and deficiency are widespread in many countries. We review the evidence pertaining to its prevention and treatment. Deficiency may be adequately treated with many different therapeutic regimens of either cholecalciferol or ergocalciferol, owing to the high therapeutic index of both compounds. Nevertheless, the current evidence suggests that regular dosing with oral cholecalciferol (e.g., 60,000 IU weekly) may have slight advantages over other regimens when replenishing vitamin D stores following deficiency. For long-term supplementation, smaller regular doses, such as cholecalciferol 1,000 IU daily, or 10,000 IU weekly, are suitable. Giving reliable and specific advice about appropriate sunlight exposure remains difficult because of differing interindividual skin pigmentation and variable sunlight UVB content at different latitudes, at different times of year, and in different terrestrial environments.