This paper is in several parts: the first presents an overview of research activities sponsored by the U.S. Nuclear Regulatory Commission (NRC) on neutron irradiation embrittlement of pressure vessel steels. A series of irradiation experiments have evaluated effects of chemistry on the fracture toughness properties of submerged arc welds representing current nuclear vessel fabrication practices and provide data to validate ASME Code practices. A correlation between Charpy energy and fracture toughness is being studied as is a program on radiation sensitivity to define the role of dose rate and to study the effects of postirradiation heat treatment (annealing). The development of a comprehensive data base for radiation embrittlement data has been initiated and is being applied in a correlation of reactor surveillance data with test reactor data. A comprehensive dosimetry effort has produced validated standard methods for calculating and measuring neutron flux and fluence parameters and the correlation of these parameters to embrittlement of pressure vessel steels in light water reactors. Our knowledge of radiation damage to reactor vessels has also been greatly increased in recent years by the accumulation of power reactor surveillance data and by better models and better analysis techniques applied to that data. Unfortunately, the results show that radiation damage is often more than previously predicted, using Revision 1 of NRC Regulatory Guide 1.99, and will result in more restrictive pressure temperature limits. The second part of this paper describes some of the system and operational impacts produced, which in turn lead to challenges to long-established safety margins. Finally, a number of outstanding issues which require further research are discussed, many of which will require better understanding of radiation damage mechanisms.