Mass Casualty events may occur as a result of natural or human-caused disasters or after an act of terrorism. The planning and response to disasters and catastrophes needs to take into consideration the distinction between progressive and sudden events. Insidious or slowly progressive disasters produce a large number of victims but over a prolonged time period, with different peaks in the severity of patients presenting to the hospital. For example, radiation events will produce a large number of victims who will present days, weeks, months, or years after exposure, depending on the dose of radiation received. The spread of a biological agent or a pandemic will produce an extremely high number of victims who will present to hospitals during days to weeks after the initial event, depending on the agent and progression of symptoms.