This article examines how lawmakers respond through law reform to the prospect of further terrorist attacks after September 11. The first part examines ways of conceptualizing the terrorist threat, drawing upon notions of risk, fear, catastrophe, and precaution. The concept of the Precautionary Principle is introduced and explored as a tool for making sense of, and improving, law reform processes in counterterrorism. The second part deploys these concepts through two case studies of counterterrorist law reform: Canada and Australia. What constitutes expertise in this area is considered, as is the capacity of legislators to scrutinize the arguments of necessity and prognoses of dire threat put forward by executive government.