This chapter introduces hazardous materials and the roles of first responders. It covers the definitions of hazardous materials and waste. Various government agencies regulate the transportation, use, and disposal of hazmat, including DOT, EPA, OSHA, and NRC. Hazmat incidents can involve thermal, radiological, asphyxiation, chemical, biological, or mechanical hazards. First responders must understand hazards and their training levels to respond safely and effectively.