Design and operation of fusion reactors will require a temperature-dependent effective toughness [Ke(T)] data base. Effective toughness is a function of intrinsic metallurgical/microstructural factors, degraded by irradiation, and extrinsic factors, such as size and geometry. Standard fracture mechanics is inadequate, since the presumption of geometrically independent crack tip stress/strain fields does not apply to either small specimens or in thin-walled structures with shallow cracks. More general approaches to measuring and applying Ke(T) data are described for cleavage initiation in steels and vanadium alloys. The critical stress-critical area (σ*/A*) mechanism of cleavage initiation is demonstrated using a confocal microscopy/fracture reconstruction method that can also directly measure Ke. The σ*/A* model is combined with finite element method (FEM) simulations of crack tip fields to: a) predict Ke(T) for F-82H as a function of size; and b) directly adjust Ke(T) data to a common test geometry. A simpler master curve-(temperature) shift method is also described. Changes in yield stress due to irradiation or strain rates can be related to the shifts. Indeed, tensile properties as a function of temperature, strain rate and alloy condition are required by all assessment methods. Physically-based small specimen methods will reduce enormously what would otherwise be a prohibitive amount of testing.