BE-2004 is the second in a series of embedded conferences that focus on generating and sustaining the dialogue regarding the use of best estimate plus uncertainty tools to license operational and advanced nuclear systems. The first conference in the series, BE-2000, was held during the 2000 American Nuclear Society Winter Meeting in Washington, D.C. Like BE-2000, BE-2004 is international in scope, as evidenced by the multinational sources of the papers, and is intended to serve as an opportunity for information exchange between research scientists, practicing engineers, and regulators. However, as appropriate to a follow-on conference, the primary theme of BE-2004 is to provide updates reflecting the progress in best estimate methodologies in the last four years. Examples include research activities that evolved from the current Generation-IV initiative and other new designs [Nuclear Energy Research Initiative (NERI), etc.], core design and neutronic calculations that support best estimate analysis, use of advanced Appendix K methodologies to produce plant licensing procedures competitive with best estimate methods, and of course current philosophical and technical issues that need to be considered in implementing best estimate codes as an established part of the international licensing framework.