NIMS states that:

• Mutual aid agreements establish the legal basis for two or more entities to share resources. Mutual aid agreements may authorize mutual aid between two or more neighboring communities, among all jurisdictions within a state, between states, between Federal agencies, and/or internationally.
• Jurisdictions should be party to agreements with the appropriate jurisdictions and/or organizations from which they expect to receive, or to which they expect to provide, assistance.

Click this link to review the Resource Management and Mutual Aid page within the NIMS Resource Center.