The basic environmental impact assessment paradigm, under the federal National Environmental Policy Act (NEPA) and state laws such as the California Environmental Quality Act (CEQA), is as follows: set forth an accurate project description, describe baseline environmental conditions at the time the project is being considered for approval, assess the impacts of the proposed project on baseline environmental conditions, and then present a reasonable range of alternatives and feasible mitigation to reduce the significant adverse impacts of the project on baseline environmental conditions. The critical temporal assumption to this basic environmental impact assessment paradigm is that appropriate alternatives and mitigation will be determined in reference to a set of baseline environmental conditions at a fixed point in time when the environmental impact assessment is being prepared.

This critical temporal assumption is found not only in environmental impact assessment laws in California and the United States but also in environmental impact assessment regimes around the world. In general, the impacts of a proposed project or policy are evaluated against the baseline conditions that exist at the time the environmental impact assessment is undertaken.