This chapter considers the utility of biochemical indicators to represent the dietary intakes of specific nutrients in epidemiologic studies. It first discusses some general principles of the relationship between nutrient intake biochemical indicators, which define the utility of these measures for epidemiologic studies. It then reviews some aspects of study design, specimen collection and storage, and measurement, relevant to the validity of biochemical indicators. The study designs available for validation of biochemical indicators as measures of intake are discussed. Finally, specific biochemical indicators used to assess intake of selected nutrients of current epidemiologic interest are considered.