This chapter reviews concepts of exposure and dose, identifies sources of contaminants, and describes the circumstances of human exposures. Occurrence of contaminants varies by setting and humans are exposed to multiple contaminants simultaneously or sequentially. Various processes act on contaminants to affect their ultimate fate and distribution within the body and the biologically effective dose relevant to the health outcome of concern. It is important to have information on sources of contaminants and the circumstances of exposure. The National Health and Nutrition Examination Survey (NHANES) contains multiple items relevant to contaminant exposure and dose. Human exposure and absorption of contaminants depend on properties of the contaminant and the medium in which it is present, as well as human activities, which change over the lifespan. The nature of reproductive system function and human development presents challenges to quantitative measurement of contaminant exposure and dose and subsequent study of health effects.