According to the Dictionary of Epidemiology, a biomarker is “a cellular or molecular indicator of exposure, health effects, or susceptibility” (1, p. 17). In this chapter, the primary focus is on markers of exposure, although the techniques described here can be applied to any type of biomarker. The process of assessing the quality of a biomarker consists of determining if the biomarker has adequate reliability and adequate validity. Reliability refers to “the degree to which the results obtained by a measurement procedure can be replicated” (1, p. 145). (Reliability is often used interchangeably with the terms repeatability and reproducibility.) The reliability of a measurement process is most often described in terms of intrarater and interrater reliability. Intrarater reliability (sometimes called intraobserver agreement) refers to the agreement between two different determinations made by the same individual and interrater reliability (sometimes called interobserver agreement) refers to the agreement between the determinations made by two different individuals. A reliable biomarker must exhibit adequate levels of both types of reliability. Also of concern in the assessment of the reliability of a biomarker are intersubject, intrasubject, and analytical measurement variability (2). The reliability of a biomarker must be established before validity can be examined; if the biomarker cannot be assumed to provide an equivalent result upon repeated determinations on the same biological material, it will not be useful for practical application.