When coping with a natural disaster, wildfire, or disease outbreak, numerous agencies at the local, state, and federal evels have to effectively communicate, coordinate operations, and allocate resources. This is equally true in the event of a serious terrorist attack. How can personnel and organizations that do not normally work together do this effectively? One solution to the potential problems of divided leadership, parallel chains of command, operational conflicts, competing resource demands, and unfamiliar professional terminology is an Incident Management System (IMS), which is designed to manage complex or multisite emergency events. This paper provides an overview of the purposes and principles of IMS.