A three-volume document on "Probabilistic accident consequence uncertainty analysis" has been published by the European Commission, DG XI and DG XII, and the US Nuclear Regulatory Commission. The document summarizes a joint project conducted by the US Nuclear Regulatory Commission and the European Commission to assess uncertainties in the MACCS and COSYMA probabilistic accident consequence codes. The codes were developed primarily for estimating the risks presented by nuclear reactors based on postulated frequencies and magnitudes of potential accidents. The project assessed uncertainty in the MACCS and COSYMA offsite radiological consequence calculations for hypothetical nuclear power plant accidents. Volume I of the document contains background information and a complete description of the joint consequence uncertainty study. Volume II contains two appendices that include the rationales for the dispersion and deposition data provided by 16 experts, the tabulated elicited information, and short biographies of the experts. Volume III contains six appendices that describe in greater detail the specific methodologies used by the atmospheric dispersion and deposition panels.