An investigation was made of a number of 0.30 and 0.40 per cent carbon alloy steels to determine the relationships between their fracture toughness properties and their internal microstructures. The plane-strain fracture toughness of the steels was measured after tempering quenched material in the temperature range 300 to 1100 F. A thin section transmission electron microscopy study was carried out on the tempered materials. From the fracture toughness studies it was concluded that all the materials behaved similarly and that alloying elements (carbon and silicon) had little influence on the general relationship between tensile strength and toughness in these fully hardenable steels. It was found that the fracture toughness remained low at low tempering temperatures but improved rapidly once a critical tempering temperature, characteristic of the particular steel, was reached. The microscopy study showed that major microstructural changes occurred in the tempering range where the rapid increase in toughness was observed. At low tempering temperatures the defect structure of the as-quenched martensite remained unchanged, and continuous films of carbide were formed in the boundaries of the martensite. In the critical tempering range the carbide films were spheroidized and the defect structure of the matrix removed or modified by recovery processes. On the basis of these observations, it was concluded that the low fracture toughness of the steels in a lightly tempered condition was due to their high defect densities and the presence of carbide films at boundaries. Only when these features were removed or modified did toughness increase.