It has often been said that, although man has built structures of and on soils since the earliest time, we probably know less about the structural properties of soils than of any other engineering materials. When thinking in terms of soil stability we can carry this axiom further by saying that we probably know less about the shear properties than any of the other properties of soils used in present-day design. Subcommittee R-5C on Methods of Testing Soils Under Triaxial Loading of ASTM Committee D-18 on Soils for Engineering Purposes has realized that the standardization of a triaxial shear test for soils may take many years because of the varied concepts in this phase of soil mechanics and foundation engineering. The subcommittee has, therefore, devoted much time to discussions of the various concepts in use today and to the testing equipment and procedures used by various soils laboratories throughout the United States. It accordingly sponsored the Symposium on Testing Soils Under Triaxial Loading, at the 1950 Annual Meeting designed to bring out some of the current thinking on these subjects. A similar symposium was held during the Annual Meeting of the Society in 1939. A comparison of the papers in the two symposiums shows that, although there have been some changes in thinking during the past eleven years, a great deal of research work still lies ahead of those interested in the subject. Because of the close similarity of problems involved in testing both soils and bituminous paving materials under triaxial loading, several papers presented in this symposium cover both the use of the triaxial test in the design of soil subgrades and bituminous mixes for highway work. In addition. three interesting papers on the applications of the triaxial test to bituminous mixtures, presented at the Society's First Pacific Area National Meeting, San Francisco, Calif., October 10, 1949, are included in this special publication. These papers were prepared by members of the Triaxial Institute, an informal group of West Coast engineers, who are also members of subcommittee B-2 of ASTM Committee D-4 on Road and Paving Materials.