Biomarkers of internal dose measure the level of a carcinogen or one of its metabolites in a tissue or a body fluid such as urine or blood. The choice of a biomarker of internal dose for a particular epidemiological study or type of study requires careful consideration of the period of exposure to which the biomarker relates, host factors related to carcinogen metabolism, invasiveness of sampling, reliability and cost of the biomarker. Before a new biomarker is adopted, it is important to assess these characteristics in transitional studies to ensure that the biomarker will be applied appropriately. Biomarkers of internal dose have been applied most successfully in ecological studies and nested case-control studies, and are especially useful when they provide information about long-term carcinogen exposure.