For the past several years the Pipeline Research Committee of the American Gas Association has sponsored research at Battelle's Columbus Laboratories with the objective of obtaining a better understanding of the behavior of defects in pressurized pipe. This objective is being pursued by means of full-scale experiments on line pipe specimens containing both artificial and actual defects. These experiments have led to the development of semiempirical equations for predicting the ductile failure stress levels of through-wall flaws and surface flaws. Although these equations have been presented before, the supporting data and analyses are described in this paper. The through-wall flaw equation is analogous to fracture mechanics criteria for plane stress fracture; but because it has been adapted to ductile line pipe materials, it contains the Dugdale Model for plastic flow in the material and a correction for the bulging stress resulting from pressure acting on the curved pipe walls. The surface flaw equation evolved from the experimental results on surface-flawed pipe specimens. It accounts for both length along the axis of the pipe, depth through the wall, and the bulging which also takes place at surface flaws. Both equations have been shown to give reliable prediction of failure stress levels for not only steel line pipe materials but for stainless steel and aluminum pressure vessels as well. The usefulness of these equations extends over a wide range of material toughness and strength levels, because they embody both tensile strength parameters and the notch-toughness as determined from the ductile shelf energy of Charpy V-notch impact specimens. The experimental results upon which the equations are based are presented and discussed herein as are the utility and degree of reliability of the equations.