A biomarker is a measurable event occurring in a biological system, such as the human body. In environmental epidemiology, a biomarker represents a subclinical and reversible change; it is not a diagnostic test, but an indicator that an early change has occurred that could later lead to clinical disease. Although some biomarkers may belong to more than one class, they are often separated into biomarkers of exposure, biomarkers of effect, and biomarkers of susceptibility. Biomarkers can be used to classify and quantify environmental exposures and their related effects, and many methods may be applicable in toxicological experiments as well as in epidemiology. Accordingly, biomarker epidemiology is undergoing rapid development and expansion and is becoming one of the most promising areas of environmental research. Although expanded applications should be encouraged, many biomarkers are poorly characterized, and attention should be paid to defining their properties in detail.