The current neutron irradiation embrittlement trend equation used in the U.S. is contained in Nuclear Regulatory Commission (NRC) Regulatory Guide 1.99, Revision 2. The database used to establish this correlation was compiled in the late 1980s. Since that time there has been about a five fold increase in data. Through the EPRI Materials Reliability Program a new and improved transition temperature shift embrittlement correlation has been developed. The recommended model is mechanistically-guided, statistically robust, and stems from earlier work on a mechanistic/statistical model proposed by the NRC. From the independent reviews performed on the NRC proposed correlation, the evaluations of mechanistic understanding and statistical testing were combined to assess the most appropriate form for a mean correlation model. The process of evaluating and reducing the number of fitting parameters was not a simple decision. Engineering judgment, through the development of gating criteria and value/magnitude considerations, led to the development of the proposed correlation. This paper discusses the proposed embrittlement correlation and its mechanistic/statistical bases.