The development of NE steels was facilitated by the previous work on the standardization of hardenability testing and on the calculation of hardenability from chemical composition and grain size. The importance of susceptibility to brittle failure has been emphasized by war experiences. Stress concentrations, low temperatures and high loading rates contribute to brittle failures. Temper brittleness, a potential cause of brittle failures, requires the determination of impact strength at a number of temperatures rather than just at room temperature. The best combinations of strength and toughness are generally found in steel which has been tempered after having been hardened to a fully martensitic structure. If present, the nature and distribution of the non-martensitic constituents are important. The decision regarding the type of structure needed in the finished part involves consideration of the needed strength and toughness, possibility of distortion and cracking in hardening and cost. The development of H steels offers the advantage of controlled hardenability. Triple alloy steels conserve alloying additions and also behave uniformly since all alloying elements are under positive control. New levels of performance seem attainable as a result of greater knowledge of the conditions necessary for maximum combinations of strength and toughness in steel.