We report a computational 'alloy by design' approach which can significantly accelerate the design process and substantially reduce the development costs. This approach allows simultaneously optimization of alloy composition and heat treatment parameters based on the integration of thermodynamic, thermo-kinetics and a genetic algorithm optimization route. Novel steel compositions and associated key heat treatment parameters are identified so as to realize the target microstructure for applications either at the room temperature (ultra-high strength maraging stainless steel) or at high temperatures (creep resistant steels). Solid solution strengthening and precipitation hardening are the two strengthening mechanisms employed to improve the strength of designed steels. Either one of them or their combination is optimized in the four steel families considered. Each model is validated by analyzing the strengthening contributions in existing steels and by experimental characterization of prototype alloys. Very good agreement between experimental performance and model predictions is found. All newly designed alloys are predicted to outperform existing high end reference grades.