Plans for disposing of radioactive waste have raised a number of unique and mostly philosophical problems, mainly due to the very long time-scales which have to be considered. While there is general agreement on disposal concepts and on the approach to establishing that disposal facilities are safe, consensus on a number of issues remains to be achieved. To assist in promoting discussion amongst international experts and in developing consensus, the IAEA established a Working Group under the International Radioactive Waste Management Advisory Committee (INWAC). This publication is the sixth report of this working group.