A method of solving the time-energy dependent problem in reactor physics is explained. This method uses an iterative process to solve the steady-state Boltzmann equation and takes advantage of the implicit time variable concealed in this equation and released by the iterative calculations. Various examples of calculations are presented for fast and thermal reactors. They reveal some interesting features of kinetics neutron population and provide information for understanding reactor kinetics and time-of-flight spectra obtained both in fast and thermal subcritical systems.