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The development of sustainable energy sources that minimize greenhouse gas emissions such as nuclear energy lessens the world’s dependence on fossil fuels while addressing climate change and longer-term sustainability. The designs of the next generation of nuclear reactors focus on high efficiency and versatility, which will require structural materials to be able to sustain much harsher environments than in current nuclear reactors. Consequently, there is an urgent need to understand the shortcomings of current materials and improve their properties as well as design more performant materials. In this two-part thesis, I am addressing the roles of alloying elements and dose rate on the irradiation response of dilute steels used in light water reactor pressure vessels (RPV) and of multi principal element alloys (MPEAs) that have shown promise as radiation resistant materials. The first part of the thesis presents a systematic experimental approach to investigate the effects of P and Ni alloying elements, and dose rate on the irradiation response of dilute RPV steels. The study revealed that P, similar to Cu, causes early nucleation of Mn-Ni-Si precipitates in neutron-irradiated RPV model steels, leading to hardening and embrittlement at early fluence. In addition, to accelerate the understanding of precipitation mechanisms in RPV surveillance steels, high dose rate ion irradiation is increasingly used, which enables the end-of-life dose (0.2 dpa) to be reached in a matter of hours to several days, compared to years required in lower dose rate test reactors. By comparing the microstructure of several RPV surveillance steels under high dose rate ion irradiation and lower dose rate neutron irradiation at similar dose levels, the study found lower precipitate number density and larger size in the case of neutrons, while the precipitate volume fraction and composition remained remarkably similar. The second part of the thesis presents the effect of composition and dose rate on the microstructure evolution of Ni-based MPEAs under irradiation and thermal aging. While prior literature claimed that Cr0.6FeNiMn and equiatomic CrFeNiCoPd possess excellent phase stability under irradiation, it is shown here that microstructure stability is controlled by dose rate, whereby phase decomposition takes place at low dose rate irradiation but is suppressed by cascade mixing at high dose rates. Furthermore, to clarify the mechanisms of phase decomposition under irradiation, phase stability under thermal conditions during aging at 500 oC was also elucidated and quantified. Furthermore, prior literature also showed high swelling resistance and suppressed damage evolution in CrFeNi-based MPEAs as compared to the less concentrated alloys under high dose rate environment (>10-3 dpa/s). To understand the effect of dose rates on the irradiation behavior of MPEAs, the microstructural responses of a series of Ni-based alloys of increasing compositional complexity were investigated after ion irradiation at the relatively low dose rates of 10-4 dpa/s and 10-5 dpa/s. The findings suggested that the effect of composition on dislocation loops and cavity swelling significantly decreased with decreasing dose rates.