Crack propagation is the one phase of fatigue failure which can be treated quantitatively without the added complication of statistical fluctuations. Moreover, in many materials, and especially in notched structures, the crack propagation phase occupies a major fraction of the useful fatigue life. There is, in fact, one school of thought which views all of fatigue damage as the growth of cracks; cracks which grow from preexisting flaws in real structures. In recent years an enormous amount of effort has been devoted to the study of fatigue crack propagation. Some of the first fruits of this effort were reported at the Crack Propagation Symposium held in Cranfield, England, in 1961. Since that time several powerful techniques for the study of crack growth have been developed more fully. Electron fractography has been applied to a wide variety of materials, and the distinguishing “fatigue striation” has been studied in detail. Transmission electron microscopy has been used for the observation of localized damage in the vicinity of cracks. The science of fracture mechanics, which treats the stress concentration, energy balance, and plasticity at crack tips, has found fruitful application to the fatigue problem. Careful microscopic observation of growing fatigue cracks has led to semiquantitative models for the crack extension process. Test apparatus for application of random loads has been developed, and crack growth under these service-simulated loads has been observed. Finally, the results of crack propagation research are becoming useful tools which the engineer can use in designing real structures. In the light of these significant advances, the members of Committees E-9 and E-24 of ASTM felt that the Society could perform a valuable service to the scientific and engineering community by bringing together representative people to review recent accomplishments in this field. This book, which details the proceedings of the Fatigue Crack Propagation Symposium, is divided into four sections. The first three sections, Fatigue Crack Growth in Structures, Microstructural Aspects of Fatigue Crack Growth, and Continuum Approach to Fatigue Crack Growth, are each introduced with a review paper which is followed by several shorter papers which report current research activities in the field. The last section of the book contains a comprehensive paper by J. Schijve, who, through his broad knowledge of the field, manages to bring together the diverse elements of crack growth research into a coherent whole. Dr. Schijve also concludes the volume with a critical survey of the results of the symposium, in which he defines those areas most in need of additional work. The last section also includes papers which report very recent experimental results which became available after the original Symposium Program had been finalized.