The Commission of the European Communities, within the framework of its 1980–1984 radiation protection research program, initiated a 2-yr project in 1983 entitled “Methods for Assessing the Radiological Impact of Accidents” (MARIA). This project was continued and enlarged within the 1985–1989 research program. The main objectives of the project are (a) to develop a new probabilistic accident consequence assessment code that is modular, incorporates the best features of those codes already in use, can be readily modified to take account of new data and model developments, and is broadly applicable within the European Communities; (b) to acquire a better understanding of the limitations of current models and to develop more rigorous approaches where necessary; and (c) to quantify the uncertainties associated with the model predictions. Approximately 120 person-yr of effort have been committed to the second phase of the project, which involves contractors from 12 different organizations and institutes in the European Communities. This has led to the development of the accident consequence code COSYMA (COde SYstem from MARIA), which will be made generally available in mid-1990. The numerous and diverse studies that have been undertaken in support of this development are described, together with indications of where further effort might be most profitably directed.