This report presents the findings of the IAEA Coordinated Research Project entitled “Natural Safety Indicators (Concentrations and Fluxes)”, the objective of which was to contribute to the assessment of the long term safety of radioactive waste disposal by means of additional safety indicators based on the observation of natural systems. The participation of countries from around the world has meant that the distribution of naturally occurring radionuclides on local, regional and global scales is now better understood. This has enabled the project to define ways in which these abundances may be used to set criteria against which the potential releases from a repository may be evaluated. This report also contains the developing views of experts within the international community and should be of use to those engaged in producing national and international standards and guidance in this area.