This chapter discusses the chemistry of the individual vitamins and understanding of the chemical and physical factors that influence vitamin retention and bioavailability in foods. It addresses the strengths and limitations of analytical methods for each vitamin. The chapter also discusses the inherent differences in stability among folates, as well as chemical and environmental variables influencing folate stability. The vitamins comprise a diverse group of organic compounds that are nutritionally essential micronutrients. Although vitamins are consumed in the form of supplements by a growing fraction of the population, in many cases the food supply generally represents the major and most critically important source of vitamin intake. To assess the impact of food composition and intake patterns on the nutritional status of individuals and populations and to determine the nutritional effects of particular food processing and handling practices, a nutritional reference standard is essential.