This study discusses the definition, determination and use of Imminent & Substantial Endangerment (ISE) included in the United States environmental laws. ISE is a contamination situation and statutory authority that state and federal governments issue an administrative order to responsible parties for clean-up. ISE is appeared in all kinds of official government documents such as administrative oders on consent, judicial consent degrees, and unilateral administrative order. In order to invoke ISE authority, the governments should have evidence of each of the following elements; 1) a possible ISE 2) because of actual or threatened release 3) of a hazardous substance 4) from a facility. The determination of ISE relies on detail scientific evidence and documentation in order to demonstrate the existence of conditions that may present an ISE. However, any official protocol or procedure has not been found for determining an ISE, although an ISE order documentation format is suggested by U.S. EPA.