A disaster can be loosely defined as an event that, over a relatively short time period, causes a large number of human injuries, deaths, and/or perhaps communicable illnesses, plus infrastructure damage. An important instance of the latter would be breakdown of an alarm, communication, or financial system, or in the disablement of a command and control system that, if in place and operative, could help manage the disaster resolution and recovery. Disasters are usually unexpected: they may come with minimal warning, and with uncertain scope and effects. Examples of natural disasters are earthquakes, fires, floods, hurricanes or typhoons, epidemics, or combinations thereof. Manmade disasters include chemical plant leakages (industrial accidents), but, importantly, chemical, biological, or other weapons of mass destruction (WMD) attacks by guerrilla terrorists on cities or military platforms or installations, conceivably within buildings, and also aircraft in flight. Multiple automobile accidents or aircraft crashes are also examples, among many others. All of these differ in detail, but share some common features.

The consequences of a disaster that we focus on first are human casualties in various categories (which include deaths), and how to minimize their number by judicious application of defensive restorative resources (service facilities: quick response diagnostic equipment and processes, relief personnel, medical supplies, other assets). We think of the disaster event as suddenly placing a demand for the servicing of a great many uncertain tasks of various urgencies. Prioritization of these tasks is the continual metatask; dynamic re-prioritization will often be required in real circumstances, but will be most effective if pre-planned and rehearsed. Astute Modeling and Simulation (M&S) of various disaster situations will clarify the force size and structures most likely to be needed, and point out the values of certain tactics and assets in an uncertain environment. Response to a disaster can probably be viewed as an emergent activity, with the victims and the providers of relief displaying dynamic interactions leading towards a goal.

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