Because tempered martensites provide some of the most desirable combinations of strength and ductility, a study was made of secondary-hardening steels—Type H-11 (Crucible 218) and Type 422, and a martensitic-hardening low-alloy high-strength steel, Type 4340—to relate fine structural aspects to strength and fracture properties. The fracture process was shown to consist of many discontinuous crack initiation, coalescence, and propagation steps merging together to produce the macroscopic fracture front. The observed large scatter in fracture toughness of air-melt 4340 in the 500-F embrittlement region was explained in terms of the high sensitivity of the fracture-toughness test to structural discontinuities at inclusion-matrix interfaces in a deformation limited matrix. Such effects were absent in vacuum-melted 4340 steels. Tempering the two secondary hardening steels at temperatures well below the secondary hardening region produced an optimum combination of strength and toughness. A fourfold toughness benefit in 422 and a substantial benefit in 218 were thus demonstrated without loss of yield strength. Tempered-martensite embrittlement, 500 F, and temper brittleness, 900 to 1000 F, were explained in terms of a unified mechanistic concept involving precipitation locking of dislocation intersections and jogs, along with high dislocation densities during carbide re-solution and reprecipitation steps.