Vitamins are organic compounds needed in minute amounts that are essential for life. A vitamin must be in the diet (dietary essential) or be synthesized by microorganisms in the digestive system and then absorbed by the host animal. Currently there are 14 recognized vitamins of which four are fat-soluble and 10 are water-soluble, but not all animals require all 14 vitamins (Table 1). When an animal absorbs an inadequate quantity of a particular vitamin, various responses are observed depending on the vitamin and the degree and duration of deficiency. The most severe situation (seldom observed in U.S. dairy cows) is a clinical deficiency. For example, rickets results from a clinical deficiency of vitamin D. Marginal deficiencies of vitamins usually have more subtle and less defined signs. Unthriftiness; reduced growth rate, milk production, or fertility; and increased prevalence of infectious diseases can be observed when animals absorb inadequate amounts of vitamins.