Temper embrittlement has been one of the perennial problems of physical metallurgy, accompanying the use of alloy steels for a number of decades. Practical solutions have been found for particular problems involving embrittlement, better methods of measuring temper embrittlement have been developed, and a great deal has been learned about this complex problem, but real understanding of the fundamental mechanisms involved has been elusive. Current trends in the design of heavy structural components, such as large pressure vessels and turbine-generator rotors, require increased size, more massive sections, higher stresses, and, in some cases, increased operating temperatures. At the same time, advances in understanding of fracture mechanics tend to require concurrent improvement in fracture toughness. Steels with higher hardenability are needed to attain the required through-section fracture toughness at the higher yield strengths needed for such components. However, temper embrittlement is assuming increasing importance as an obstacle inhibiting progress in the design of such heavy components. The higher-alloy steels required for through-section hardenability and toughness tend to be much more susceptible to temper embrittlement than the lower-alloy pearlitic steels. Larger ingots imply greater segregation of alloying and embrittling elements, and more massive sections must be cooled more slowly through the temperature range of embrittlement. Susceptible steels operated for long times within the temperature range of embrittlement, 350 to 575 C, may embrittle to a surprising degree; the notch toughness transition temperature may increase by hundreds of degrees. The possibility of such embrittlement must be considered in every phase of design, heat treatment, and operation if unexpected deficiencies or losses in fracture toughness are to be avoided. The technical and economic value of effective control of temper embrittlement in high hardenability steel is very great.