This chapter discusses two vital legal weapons that were made available to the authorities to deal with dangers posed by terrorists in the wake of the September 11 attacks in the United States and the many bombing tragedies in a number of countries. It provides an account of how preventative detention measures were considered by the courts in Australia and the United Kingdom. Issues relating to the constitutional validity of preventative detention orders and control orders provided by federal legislation are canvassed by reference to the separation of judicial power doctrine. The persona designata doctrine and the incompatibility doctrine as expounded by the High Court are discussed. Particular attention is given to the landmark case of Thomas v. Mowbray, in which the validity of a control order authorised by a federal judge was upheld by the High Court. Attention is focused on the operation of the Kable principle to determine the validity of state legislation authorising preventative detention orders and control orders.