In the event of a serious terrorist attack, numerous agencies at the local, State, and Federal levels have to effectively communicate, coordinate operations, and allocate resources. The IMS is designed to manage complex or multisite emergency events. An incident commander or a unified command team is responsible for the successful resolution of the emergency through a process of authority delegation and coordination among many participating agencies. IMS creates a clear chain of authority that can quickly orchestrate collaborative operations by diverse organizations that have had little or no previous operational relationships. IMS has the ability to accommodate small and large events, but is most important when a large-scale incident occurs. The State of California pioneered the Incident Command System, later renamed the IMS, when its firefighting resources were severely taxed by major wildfire outbreaks. The problems experienced were no clear-cut leader or incident manager, no collaborative organizational structure, no common terminology, no joint communications system, and no system for allocating scarce resources. This original system was designed for wildland fire operations but was later adopted by the urban fire service and spread to other States. The IMS establishes four distinct functional subunits: operations, logistics, planning, and finance/administration. In a disaster management system, the concept of span of control is important. It refers to the number of people that a single supervisor can successfully manage and coordinate. An IMS is therefore tiered. The organizational layers are used only when appropriate and utilized only as dictated by system complexity. Staffing and training are central issues of IMS operability, particularly at the local, division, and facility levels. 1 figure, 2 footnotes, 2 appendices