A model is developed for the study of fast reactor flux and importance spectra. The model is based on slowing down theories of the continuous slowing down type with suitable modifications introduced where such theories are deficient. The ability to obtain analytic solution for the flux and importance spectra depends on the ability to fit basic cross-section ratios (such as the moderating ratio) to simple analytic forms. The cross-section fits are used to obtain an expression for the slowing down density, which generally is a smooth function. Detailed cross sections, however, are used to relate flux and importance to slowing down density. The model yields good agreement with multigroup spectra on both broad group and ultrafine group bases and yields quite accurate values for integral parameters (e.g., criticality and reaction rates). In addition, it is found possible to obtain a good correlation of cross-section fit parameters with composition.