Since the dawn of the Atomic Age, nuclear experts have labored to imagine the unimaginable and prevent it. They confronted a deceptively simple question: When is a reactor 'safe enough' to adequately protect the public from catastrophe? Some experts sought a deceptively simple answer: an estimate that the odds of a major accident were, literally, a million to one. Far from simple, this search to quantify accident risk proved to be a tremendously complex and controversial endeavour, one that altered the very notion of safety in nuclear power and beyond. This book traces these contentious efforts, following the Atomic Energy Commission and the Nuclear Regulatory Commission as their experts experimented with tools to quantify accident risk for use in regulation and to persuade the public of nuclear power's safety.