ASTM E900-07, Standard Guide for Predicting Radiation-Induced Transition Temperature Shift in Reactor Vessel Materials, includes a transition temperature shift (TTS) equation that predicts the effect of neutron radiation on the embrittlement of ferritic reactor pressure vessel steels. In E900 TTS is quantified by the shift in the Charpy V-notch transition curve at 41 J of absorbed energy (ΔT_41J ). The TTS equation in E900-07 was first adopted in the 2002 revision. In 2011 Subcommittee E10.02 began an extensive effort to evaluate the adequacy of this equation for continued use. This effort, which is summarized herein, aimed to assess how well the TTS equation represents the effects of composition (copper, nickel, phosphorous, manganese, silicon, etc.), exposure (fluence, flux, temperature, etc.), and categorical (product form, welding flux type, manufacturer) variables on ΔT_41J . The subcommittee recognized that this effort might reveal the TTS equation to be outdated, so the evaluation also considered eight other TTS equations published since 2002. The initial goal was to recommend either the current E900 TTS equation or one of the newer equations for use in what became the 2015 version of E900. For reasons described herein this was not possible, so the effort was expanded to develop a new TTS equation. This paper provides a synopsis of the effort to develop the new TTS equation. The complete technical basis for this development effort is documented in an ASTM adjunct.