Failure in soils often means that one mass of soil begins sliding over another mass. The soil is said to fail in shear because the movement at the failure surface involves relative displacement parallel to the failure surface (Fig. 1). Several types of laboratory tests (direct shear, triaxial shear) were developed during the 1930's to measure the ability of soil to resist shearing stresses. These tests are still by far the most widely used and will be the main tests discussed in following chapters.