This paper presents a systematic review of the behavior of phosphorus (P), highlighting the implications of P segregation to grain boundaries under neutron irradiation. The review focuses on Mn-Mo-Ni steels employed in US pressurized water reactors (PWRs), and other PWRs worldwide. Segregation of P to grain boundaries in reactor pressure vessel (RPV) steels can occur during fabrication (especially during the slow cooling stage of a post-weld heat treatment), and as a result of in-service exposure to high operating temperature and irradiation. This segregation of P to grain boundaries can promote a change in the brittle fracture mode from transgranular (TGF) to intergranular (IGF), and a degradation in the mechanical properties. In US RPV steels, most data are on thermal aging of the heat-affected zone (HAZ). Studies in coarse-grained HAZ have shown that the embrittlement arising from segregation of P to grain boundaries is approximately linearly related to the proportion of the brittle fracture that is IGF, and/or the P concentration at the grain boundary. Data are sparse on the effect of irradiation at 288°C on P segregation, and on the contribution of IGF to the total shift in the 41J transition temperature, T41J. In general, the bulk P content appears to be less than about 0.028 wt% P, with base metals having lower levels than weldments. In addition, the consequences of vessel annealing are considered at temperatures around 475°C. It is certain that the annealing treatment will have the consequence of reducing the irradiation hardening, but may significantly increase the grain boundary phosphorus coverage and the likelihood of IGF.