By analytical investigation of tissues and body fluids as well as of metabolites it is possible to evaluate certain substances that are considered “bioindicators”. With a generic term these substances have been denominated "biomarkers" (biological markers). One category of biomarkers that is gaining an increased interest are the nutritional biomarkers. A nutritional biomarker can be any biological specimen that is an indicator of nutritional status with respect to intake or metabolism of dietary constituents. It can be a biochemical, functional or clinical index of status of an essential nutrient or other dietary constituent. Nutritional biomarkers are used for a variety of purposes in large-scale population surveys and epidemiologic studies as well as smaller clinical studies. The main reasons for using nutritional biomarkers are to provide measures of nutritional status that have less error than dietary data, nutrient status for nutrients with inadequate dietary data, to obtain a more proximal and integrated assessment of nutrient status that incorporates metabolism, to assess dietary change and compliance in intervention studies, and dietary intake for the validation of dietary questionnaires.