The effects of alloy content and heat treatment on the strength and toughness of 9Ni-4Co alloy steels are discussed in detail. The strength was determined with standard tension specimens, while toughness was evaluated with various accepted, sharply notched specimen types. Of the alloying elements used, carbon acted as the strongest embrittler but was necessary as a strengthener. Silicon and the carbide-forming elements decreased toughness, particularly at higher carbon contents. Hence, the use of silicon at other than residual levels was undesirable. The addition of carbide-forming elements in appreciable amounts needed careful consideration. Nickel was used in substantial amounts to improve toughness, while cobalt was added to inhibit retained austenite formation consequent upon the high nickel content. Cobalt enhanced self-tempering effects, particularly for the lower carbon varieties. Heat treatment strongly influenced the properties as a result of the attendant changes in the microstructure. Isothermal transformation to lower bainite enhanced toughness in the 0.45%C, 9%Ni-4% Co high performance (HP) 9-4-45 steel. The toughness was superior to that of tempered martensite at equivalent strengths. The properties of upper bainite were not as attractive as those of lower bainite. Mixed structures composed of bainite and martensite lowered toughness. The two commercial grades of the 9Ni-4Co alloy system, HP 9-4-25 and HP 9-4-45, had a better combination of strength and toughness than other carbon-strengthened steels. The self-tempering characteristics of the HP 9-4-25 grade and its inherent toughness contributed to high joint efficiences and formability in the as-welded condition. No post-heat treatment of the welded joint was required.