The National Environmental Policy Act (NEPA) of 1969 implementing regulations require an assessment of cumulative impacts in reaching a determination regarding the potential significance of impacts associated with a proposed action. Analysis of cumulative impacts is one of NEPA's most challenging and complex requirements. With respect to the assessment of cumulative impacts, a strict regulatory interpretation of “significance” can lead to a paradox (the Cumulative Impact Paradox, or “Paradox”). A Finding of No Significant Impact (FONSI), by its very definition, states that an action will not result in a significant effect, including a cumulatively significant impact. But because many environmental resources have already sustained significant cumulative impacts, a logical paradox can arise in which a strict interpretation of the regulatory requirements leads to the conclusion that many federal activities are technically ineligible for FONSIs and therefore require preparation of Environmental Impact Statements. A strict interpretation of “significance” can lead to such a conclusion, even in cases where the direct and indirect impacts of the proposed activity may be relatively nonsignificant. This Paradox must be resolved, if the analysis of cumulative impacts is to be practically and effectively integrated into federal decision making. This article presents a method or tool referred to as the Significant Departure Principle for resolving the aforementioned Paradox. Emphasis is placed on assessing environmental resources that have already sustained cumulatively significant impacts.