FOREWORD Models are essential tools for evaluating radiological impacts within the safety assessment process and for regulatory control of nuclear facilities and activities in planned, existing and emergency exposure situations. Modelling the fate of radionuclides in the environment and assessing the resulting radiation doses to people and the environment is needed, for example, for evaluating the radiological relevance of routine and accidental releases of radionuclides, for decision making during remediation activities, in the framework of long term safety assessments of nuclear waste disposal facilities, and for clearance and exemption of material with low levels of radioactivity from the need for regulatory control. The IAEA has been organizing programmes of international model testing since the 1980s. These programmes have contributed to a general improvement in models, in the transfer of data and in the capabilities of modellers in Member States. IAEA publications on this subject over the past three decades demonstrate the comprehensive nature of the programmes and record the associated advances that have been made. From 2012 to 2015, the IAEA implemented the Modelling and Data for Radiological Impact Assessments (MODARIA) programme, which concentrated on testing the performance of models; developing and improving models for particular environments; reaching consensus on datasets that are generally applicable in environmental transfer models; and providing an international forum for the exchange of experience, ideas and research information. Different aspects were addressed by ten working groups within MODARIA, covering four thematic areas: remediation of contaminated areas; uncertainties and variability; exposures and effects on biota; and marine modelling. This publication describes the work of Working Group 10, Modelling of Marine Dispersion and Transfer of Radionuclides Accidentally Released from Land-based Facilities. The IAEA is grateful to all those who participated in Working Group 10, in particular its leader, R. Periá?ez (Spain). The IAEA officers responsible for this publication were P. McGinnity of the IAEA Environment Laboratories, and A. Kennedy and D. Telleria of the Division of Radiation, Transport and Waste Safety.