An approach for the description and evaluation of the uncertainties in the predicted consequences of fast breeder reactor accidents is outlined. All uncertainties are viewed as uncertainties of parameters, either of physical parameters or of quantities that appear in the parameterization of phenomenological models. By the incorporation of parameter uncertainties in the simulation of the accident progression, single accident-path scenarios assume the character of “accident spectra.” The progression of accident spectra is found by combining deterministically calculated accident-path results with the probability of the respective set of input parameters. The substantial method development needed for the implementation of the approach is discussed, and the status of the development is briefly reviewed. Typical results are presented for illustration purposes. The possible eventual significance of the approach is indicated.