In this chapter, we begin with how the World Health Organization (WHO) and the United States define environmental health. We then turn to describing environmental hazards and how to measure and assess environmental exposures. Next, we examine the process through which hazardous agents enter and influence body systems, and why individuals respond differently to toxic exposures. Then, we show the relationship between exposure and disease risk assessment. We conclude with case studies of two toxic agents—mercury and dioxin—that demonstrate how the concepts provided in this chapter can be applied.