Fracture toughness is an indication of the resistance of a material to physical separation by a process of unstable macrocrack propagation. Conceptually, it is an intrinsic material parameter that should not vary with changes in specimen size, speed of loading, temperature, etc. The brittle-ductile transition size effect of metal specimens is discussed using the strain energy density theory. By assuming that the critical strain energy factor, Scr, is a material constant, predictions on the fracture behavior of metals can be made. The following remarks are helpful toward defining a fracture toughness parameter associated with instability of the macrocrack: 1. The dominant or continuum crack travels in the elastic portion of the material always skirting around or bypassing the yield material and hence releases only elastic energy. 2. Stable macrocrack growth in metals is the result of necking or constraint from specimen boundary. 3. The last ligament of macrofracture will terminate in a plane inclined to the free surface or boundary of the specimen.