Hazards can be natural, technological, or man-made. Natural hazards include weather extremes such as hurricanes and heavy rainfall, hydrological extremes such as floods (which may be influenced by engineered systems, such as dams, levees, and seawalls), earthquakes, and tsunamis. Accidental leakage of poisonous gas from chemical plants or radioactive spills from nuclear power plants are examples of technological hazards. Man-made hazards include malicious attacks (terrorist attacks), intentional sabotage, and warfare. Besides predictability, hazards differ in early warning lead times, which translate to the ability of communities to get prepared. Furthermore, certain hazards such as weather extremes and terrorism are more influenced by global change relative to other hazards such as earthquakes. This chapter discusses different hazard categories, the nature of their evolution, and approaches to understand and model these hazards. The extent to which a hazard turns into disaster depends on exposure and vulnerability, which are discussed here and in other chapters.