This chapter describes the characterization of risk and the application of risk assessment methodologies by governments, corporations, and special interest groups so as to identify the nature and extent of known and potential risks to public health and welfare from exposure to environmental chemicals. It discusses the nature of risk, including perceived vs. real rick, risk identification and quantification, and risk communication. It reviews the different approaches that may be used in risk assessment and provides specific applications of risk analysis, including the setting of exposure limits and/or emission limits and the specification of control technologies to reduce emissions of chemicals.