Waste is usually defined as something to be discarded as unwanted, unusable, or as a byproduct. There are several broad categories of waste generally recognized: industrial waste, municipal (or household) waste, and hazardous or special wastes such as radioactive and biohazardous waste. Waste management refers to the governance and handling of solid, liquid, and gaseous materials externalized by some actor, using one or a combination of strategies that include air pollution abatement, waste water treatment, landfilling, incineration, recycling, biodegradation, and waste trading. Patterns of waste management vary geographically and also involve transboundary movements of waste. Technological and economic shifts have led to the growth and centralization of the waste management industry in many countries, although scavenging and community waste management persist in places. Research into waste management relates to environmental justice, individual environmental behavior and citizenship, industrial ecology and the “afterlives” of products, as well as the ethics of waste.