The renewed interest in nuclear energy triggered by concerns about global climate change and security of supply could lead to substantial growth in nuclear electricity generation and expanded interest in fast neutron reactors with closed fuel cycles. Moving from the current fleet of thermal neutron reactors to fast neutron systems will require many decades and extensive RD&D efforts. This book identifies and analyses key strategic and policy issues raised by such a transition, and provides guidance to decision makers on the best approaches for implementing transition scenarios.