Editors' Summary: The worst case analysis regulation has brought agencies, litigants, and courts to examine NEPA's requirements for dealing with uncertainty in decisionmaking. Since our last Comment on the regulation, over a year ago, the case law has grown and the outlines of agency obligations to address uncertainty have emerged. The author explores three basic questions about the regulation in light of the developing case law: What level of uncertainty triggers the worst case requirement? When must the analysis be done? And what is the worst case? The Comment concludes that an agency may reach a fair answer to each question by applying the "rule of reason" common to NEPA litigation to construction of the regulation.