COSYMA (COde SYstem from MARIA) is a new program package for assessing the off-site consequences of accidental releases of radioactive material to atmosphere, developed as part of the Commission of the European Communities Methods for Assessing the Radiological Impact of Accidents (MARIA) program. It represents a fusion of ideas and modules from the program system UFOMOD from Kernforschungszentrum Karlsruhe, the program MARC from the National Radiological Protection Board, new model developments, and data libraries from other MARIA contractors. The flexible coding permits a problem-oriented application to different sites, source terms, emergency plans, and needs of users in the various parts of Europe. An overview is given of the structure, models, and endpoints of COSYMA.