The vitamin D system has been strongly conserved in evolution. It links sun exposure to a multitude of endocrine messages. In most body cells the active hormone calcitriol binds intracellularly to the vitamin D receptor and regulates the expression of specific gene products. Vitamin D deficiency is epidemic affecting some 1 billion people worldwide and is mainly caused by chronically inadequate sun exposure. This deficiency is associated with harmful effects on almost all tissues including a predisposition to cancer. In cancer patients vitamin D deficiency is associated with a worsening of the prognosis. The active hormone calcitriol has anti-tumor activity and is being investigated as an anticancer agent. There is general agreement that exposure to sunlight should be increased while carefully avoiding UV erythema. In addition, recent suggestions call for a health-promoting dietary intake of 25-100 mg (1,000-4,000 IU) of vitamin D3. While supplements of vitamin D improve musculoskeletal symptoms, proof is still lacking that these doses convey a protection from cancer. Interventional studies that administer vitamin D versus placebo in patients with cancer should be a high priority because of the hypothesized benefits and the low risk of supplementation with vitamin D.