The purpose of this monograph is to examine the concepts and to identify the principles for the application of biomarkers to assessment of risk to human health from exposure to chemical agents, with special attention to criteria for selection and validation.The term "biornarker" is used in this monograph, as it is in the US National Academy of Sciences report (US NRC, 1989b), in a broad sense to include almost any measurement reflecting an interaction between a biological system and a potential hazard, which may be chemical, physical or biological.