Vitamin D is a secosteroid molecule which, in its active 1,25 di-hydroxylated form, has hormone activities in humans. Most cells and tissues in the body have vitamin D receptors (VDRs) that stimulate the nuclear transcription of various genes to alter cellular function or provide a rapid response in cellular membranes. Vitamin D appears to have an effect on numerous disease states and disorders, including chronic musculoskeletal pain, diabetes (types 1 and 2), multiple sclerosis, cardiovascular disease, osteoporosis, and cancers of the breast, prostate, and colon. According to many researchers there is currently a worldwide vitamin D deficiency in various populations, including infants, pregnant and lactating women, the elderly, individuals living in latitudes far from the equator, persons who avoid the sun or ultraviolet radiation in the blue spectrum (UVB), and populations with dark skin pigmentation. Vitamin D in the food supply is limited and most often inadequate to prevent deficiencies. Supplemental vitamin D is likely necessary to avoid deficiency in winter months; however, all forms of vitamin D supplementation may not be equal in efficacy for maintaining optimal blood levels.