An assessment of tritium breeding requirements for fusion power reactors is presented. The analysis is based on an evaluation of time-dependent tritium inventories in the reactor system. The method presented can be applied to any fusion systems operating in a steady-state mode as well as in a pulsed mode. As an example, the UWMAK-I design was analyzed and it has been found that the startup inventory requirement calculated by the present method significantly differs from those previously calculated. The effect of reactor parameter changes on the required tritium breeding ratio is also analyzed for a variety of reactor operation scenarios. A FORTRAN-IV computer program, the JET-I code, has been developed to implement the present method. It is now in operation on the Argonne National Laboratory IBM-3033 computer system.