An overview is given of biological markers of exposure to chemical warfare agents. Metabolites, protein, and/or DNA adducts have been identified for most nerve agents and vesicants and validated in experimental animals or in a small number of human exposures. For several agents, metabolites derived from hydrolysis are unsatisfactory biomarkers of exposure because of background levels in the human population. These are assumed to result from environmental exposure to commercial products that contain these hydrolysis products or chemicals that are metabolized to them. In these cases, metabolites derived from glutathione pathways, or covalent adducts with proteins or DNA, provide more definitive biomarkers. Biomarkers for cyanide and phosgene are unsatisfactory as indicators of chemical warfare exposure because of other sources of these chemicals or their metabolites.