5 ILCS 235/10
In order to more adequately address emergencies that extend or exceed a jurisdiction's emergency response capabilities, either without rising to the level of a state or local declaration of a state of emergency, or in the initial stages of an event which may later become a declared emergency, a political subdivision of this State, including a county, city, village, township, or other unit of local government, or any combination thereof, may enter into a mutual aid agreement with one or more units of government from another state. The mutual aid agreement may provide for coordination of communications, staging, training, and response to planned and unplanned events which a local jurisdiction has determined exceed, or are likely to exceed, its emergency response capabilities. When engaged in training, staging, and emergency response in accordance with the mutual aid agreements, emergency responders from outside of this State are permitted to provide services within this State in accordance with this Act and the terms of the mutual aid agreement.
5 ILCS 235/10