This study on transitioning from thermal to fast neutron nuclear systems was carried out under the auspices of the Committee for Technical and Economic Studies on Nuclear Development and the Fuel Cycle (NDC). It benefitted from previous and ongoing activities on scientific and technical aspects of transition scenarios undertaken under the leadership of the Nuclear Science Committee. Its main objectives were to provide insights on strategic and policy issues raised by the transition, and to draw findings, conclusions and recommendations for consideration by decision makers on the opportunities and challenges associated with the introduction of fast neutron systems in a fleet of thermal neutron reactors.