Book excerpt: The authors document and define many of the key health related roles of vitamin D. Its traditional application in bone and muscle health as well as therapy of arthritis is expanded and clarified with new research. A better understanding of the effects of vitamin D inadequacy is modelled using problems ranging from infant growth retardation to chronic kidney and periodontal disease. Uniquely the vitamin's role in resistance and treatment of infectious diseases is shown in examples ranging from HIV/AIDS to tuberculosis. Mechanistic understanding of vitamin D's actions is enhanced by looking into its effects on immune modulation and inflammation. Expansion of the role of sunlight in stimulating vitamin D production is discussed relative to the reduction in a variety of cancers.