THE MASTER CURVE CONCEPT HAS INTRODUCED TEST PRACTICES AND an analysis methodology that has made fracture mechanics, as applied to structural steels, a practical technology. Specimen size requirements are now practical for routine laboratory testing purposes. Data developed with small specimens can be scaled to predict the fracture toughness behavior in applications that involve other sizes or dimensions. In addition, statistical methods have been introduced that model the data scatter displayed by structural steels in the transition range. Most important, a minimum number of replicate tests is necessary. This allows for the prediction of the lowest fracture toughness likely to be encountered, which is of use to establish design safety margins in terms of critical flaw sizes and/or design stresses.