The work described in this thesis details the design and characteristics of low-Ni maraging steels suitable for use at elevated temperatures. In addition to having a stable microstructure, they require strength, ductility, fatigue- and creep-resistance with low impurity levels suitable for use in aeroengine shafts. Two chemical compositions have been designed based on thermodynamic calculations which independently investigate the suitability of the matrix and the strengthening phases for use at elevated temperatures. It is believed that substituting Ni with Cr will increase the austenite-start temperature, thus retarding the formation of austenite during manufacture and simulated service at 450°C. Avoiding the formation austenite concomitantly with precipitate coarsening is important when designing a suitable shaft material. The designed alloys have exhibited an excellent combination of strength and ductility in the hardened condition achieved through a non-scale distribution of Ni-rich particles. These particles resist growth at 450°C while largely maintaining the mechanical properties. The austenite observed in one alloy transformed to martensite at room temperature during plastic deformation increasing the elongation. However, the increased thermodynamic stability of austenite at 450°C does not permit the martensitic transformation, thus it must be avoided in shaft alloys. The alloy which resisted the formation of austenite displayed excellent fatigue- and creep-resistance in the hardened condition. These properties are attributed to the size and dispersion of the strengthening phases and the level of inclusions within the microstructures. Neither of the designed alloys meets all the mechanical property requirements of the shaft. However, by combining the desirable features of both alloys it is believed that these properties can be achieved though a low-Ni maraging steel.