Extensive research has been devoted to the development of biomarkers of environmental and occupational exposure to lead (Pb). This body of work can serve as a paradigm for biomarker development for other chemical exposures. Early efforts focused on indirect measurements of exposure by analyzing precursors and enzymes of a biosynthetic pathway (heme) in blood and urine. However, the direct measurement of Pb in blood has become increasingly simple and reliable and is now widely accepted for pediatric surveillance programs, in part because of known associations of Pb with adverse health outcomes. Other markers of exposure include measurements of Pb in important compartments: bone Pb, tooth Pb, and chelatable Pb. In addition, the technique of stable isotope dilution is available, since Pb exists in numerous nonradioactive isotopic forms. The strengths and weaknesses of all Pb biomarkers for confirming a diagnosis or for epidemiologic research vary widely depending upon the hypothesis under investigation.