Exposure biomarkers, which have long been restricted to the framework of occupational hygiene, currently arouse increasing interest in the field of environmental pollution. To assess their validity, we propose here a conceptual framework that is based on their intrinsic characteristics and on properties related to the procedures for their analysis. The most important criteria are specificity for the toxic substance under consideration and sensitivity, that is, the ability to distinguish contrasted levels of exposure. Their analytic sensitivity and specificity are also important. Fulfilling these criteria is especially important in the context of environmental pollution, because the levels of exposure, and thus the contrasts, are low. This framework is used to assess the validity of some biomarkers for polycyclic aromatic hydrocarbons (1-hydroxypyrene and DNA adducts) and for benzene (urinary and serum benzene, trans,trans muconic acid, and S-phenylmercapturic acid). This evaluation shows that the most relevant biomarkers for estimating individual exposure to environmental pollution are 1-hydroxypyrene for polycyclic aromatic hydrocarbons and urinary benzene and S-phenylmercapturic for benzene.