Abstract The interrelationship between prior austenite grain size and age hardening is examined in 18Ni maraging steels. It is shown that the Hall-Petch relationship is obeyed independently of the heat-treatment condition or alloy content. Although the yield strength of unaged 18Ni maraging steels is almost independent of prior austenite grain size, i.e., k y is quite low, in the aged condition the grain size-yield strength dependence approaches that of pure iron. These differences are related to (a) the presence of a dislocated lath substructure in unaged maraging steel, and (b) the effects of solute content variation on the Hall-Petch parameters, k y and σ o. Finally, it is shown that coarse grained, high strength maraging steels can fail catastrophically. This failure involves intergranular dimple crack propagation, the latter depending upon the attainment of a critical fracture stress within the near grain boundary region.