What is meant by exposure assessment? How is exposure assessment conducted, and, in particular, what is a biologic marker? How does exposure differ from dose? In answering these questions, this chapter describes the public health importance of assessing the extent and nature of human exposure to hazardous substances released from uncontrolled hazardous waste sites. This chapter is organized around seven kinds of exposure assessment data and the value of each. Emphasis is given to the exposure properties of the 30 substances found most often in NPL sites that have completed exposure pathways. Reference data for these priority substances and key information gaps in knowledge are described.