Research to understand the mechanisms of neutron irradiation embrittlement of reactor pressure vessel steels has assisted in the development of improved radiation damage trend curves such as Regulatory Guide 1.99, Revision 2. Statistically calibrated, physically-based models of the embrittlement process have been derived which include the effects of variables such as copper, nickel, and fluence. The ability to accurately predict the amount of damage or embrittlement of the vessel is essential to the safe, continued operation of the plant. Operating criteria, such as the pressure and temperature limits, are determined for each plant in accordance with the margins established in Section III of the ASME Code. These curves must be updated periodically to account for the effects of embrittlement in terms of the nil-ductility reference temperature (RT^NDT) shift. The application of the improved radiation damage trend curves, and the impact on plant operations, are discussed in light of the recent studies of the embrittlement phenomena.