A review of data generated utilizing ASTM Method E 399 since its last substantial revision in 1971 illustrates the importance of most of the major provisions and criteria for validity, but also that some technical revisions of the criteria are justified. For the standard geometry of specimen (a = B), the P_max/P_Q criterion is a useful indicator of plane-strain crack resistance curve characteristics and should not be “stretched” or ignored in establishing the validity of test data. Alternative geometries are also useful in measuring K_Ic so long as B >¯ 2.5 (K_Ic/σ_ys)², but the present P_max/P_Q limit is too severe for W/B > 2; there is a need to establish alternative limiting values of P_max/P_Q to go with the alternative geometries. With regard to fatigue precracking, the K_fmax during fatigue precracking could be increased from 0.6 to 0.8 K_Ic, and the limits on fatigue crack front straightness could be modified to permit differences of as much as 10 percent among the middle three measurements.