The perspective presented here underscores the part played by grain size, grain boundary chemistry and substructural features such as martensite lath and packet size in determining the fracture toughness of high strength iron-based metallic materials. Each of these features influences the degree of plasticity. The notion of grain boundary segregation isotherms and thermodynamics related to the systems is explained, along with the effects on toughness of segregation of alloying elements to grain boundaries in complex alloy systems, specifically precipitation hardened ferrous alloys and maraging steels. The extensive review of experimental metal processing technology has provided a means of analysing the relationship of toughness to microstructural features. Important instances include maraging steels, precipitation hardened stainless steels, low alloy steels, interstitial-free steels, and microalloyed steels.