There are two basic aspects to consider in an exposure assessment-the first is what happens to chemicals in the environment and the second is what behaviors of the receptors produce contact with potentially contaminated environmental media. Thus, exposure assessment for both human health risk assessment requires consideration of both physical-chemical aspects of contamination (how chemicals occur in environmental media) and human behavior (how people come in contact with these media). These two aspects will be referred to as the environmental and behavioral realms. Aspects of exposure in ecological risk assessment will be considered in Chapter 6. The need to obtain data that address questions in both these realms and the resulting use of a number of assumptions introduces uncertainty into exposure assessment-evident in numerous examples in this chapter.