A hazardous waste is one that is ignitable, corrosive, reactive, toxic, or more than one of these. As with municipal solid waste, hazardous waste is also a small percentage of the 10 billion tons (9.1 billion tonnes) of total wastes that the United States generates each year. But, hazardous waste has been abandoned at many thousands of sites in the United States, and around the world. At these sites, hazardous substances evaporate into the air, contaminate soil, seep into groundwater or run off into nearby water. Section I of this chapter overviews hazardous waste, its characteristics, locales of hazardous-waste sites, and who generates hazardous-waste. Section II takes us to the waste-management hierarchy as applied to hazardous waste. Section III deals with hazardous-waste sites, evaluating their risk, and how human exposure occurs. Section IV moves on to reducing the risk of hazardous-waste sites. It examines clean-up methods including bioremediation. Section V takes us to hazardous-waste dumping in impoverished locales and the international accord negotiated to combat this practice. One waste still moving into developing countries is electronic discards, a steadily rising waste stream. We look at how we can reduce that flow.