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Nuclear power is considered to be a reliable way to satisfy the fast growing energy demand while reducing greenhouse gas emissions. However, many material challenges need to be addressed to safely extend the operational lifetime of current reactors and to develop advanced new reactors with maximised performance efficiency. This is due to the extreme reactor environment, of high temperatures and irradiation, that many components must withstand in-service. Iron-based alloys, especially Fe-Cr based steels are widely used as structural materials in current reactors and are proposed as core materials in the design of the next generation of reactor systems. To this end, using atom probe tomography (APT), this study investigates the atomic-scale microstructural changes within two Fe-Cr based steels, 17-4PH steel and T91 steel, subject to heat treatment and irradiation. 17-4PH steel is heat treated at two different temperatures, 480 °C and 590 °C for times up to 1000 hours and 24 hours respectively. The sequence of microstructural changes at the atomic scale in a 17-4PH steel is characterized by APT. At the lower temperature, the segregation of NbN/CrN ionic species, Cu-rich precipitates (CRPs), Nb-rich precipitates, Cr-rich precipitates and ultimately a Mn, Ni, and Si-rich (MNS) phase was observed to form, the latter two of which were not observed at the higher temperature. The evolution in number density and fraction of CRPs and Cr-rich α' phase, has been quantified and a simple model has been developed to estimate their respective contributions to the overall precipitation hardening of the material. As part of a US-UK Integrated Research Project (IRP) project, another aim of this study is to demonstrate the capability to predict the in-reactor evolution of T91 microstructure at high doses, using ion irradiation as a surrogate for neutrons. APT has characterized the microstructural changes in T91 steel for different irradiation conditions, single beam (Fe++) irradiation, dual beam irradiation (Fe++ and He++) and BOR-60 reactor irradiation. Irradiation-induced precipitation, grain boundary segregation and modification of pre-existing precipitates has been directly compared between dual ion beam irradiation and reactor irradiation. APT characterisation together with complementary chemiSTEM shows that dual beam irradiation can be calibrated such that the atomic scale microstructural changes are in relatively good agreement with those caused by the corresponding reactor irradiation conditions.