Confidence in Models of Radionuclide Transport for Site specific Assessment : Workshop Proceedings Carlsbad New Mexico United States 14 17 June 1999 GEOTRAP is the OECD NEA Project on Radionuclide Migration in Geologic Heterogeneous Media carried out in the context of site evaluation and safety assessment of deep repository systems for long lived radioactive waste.Performance assessment of proposed waste disposal sites requires models of radionuclide transport through the geosphere. To be used in repository planning and development these models must have the confidence of both national waste management programmes and the wider scientific community. The fourth GEOTRAP workshop Confidence in Models of Radionuclide Transport for Site specific Performance Assessment held in June 1999 addressed the issue of technical confidence building and provided an overview of current developments in this field. Proposed approaches to confidence building and approaches that have already proven successful were presented and discussed. In addition to the material presented during the workshop this publication includes a technical synthesis reflecting the discussions that took place as well as the conclusions and recommendations made notably during the working group sessions.