The primary source of vitamin D is the skin, following exposure to ultraviolet radiation. Vitamin D is well known for its effects on stimulating calcium absorption and is thus essential for maintenance of normal bone. It is also important for muscle function and has more recently been implicated in protection against several diseases including diabetes. Different pathways of action have been described for vitamin D compounds and various analogs specific to these pathways have demonstrated potential for therapeutic use. Recent studies suggest a novel role for vitamin D compounds in protection against cancer, a proposal supported by substantial epidemiological evidence.