An important objective of the civil and mining engineer concerned with structures in rock such as tunnels, open pits, dam foundations and so forth is the creation of a safe and efficient design. This necessitates an estimate of the immediate and long-term performance of the surface or subsurface opening or foundation, whichever the case may be. This in turn requires a quantitative estimate of those physical and mechanical properties of the rock mass which govern its strength, permeability and deformation charac­ teristics. To a lesser or greater extent, depending upon the type of engineer­ ing structure considered, these properties are a function of the 1) attitude, 2) geometry and 3) spatial distribution of the joints and other discontinuities in the mass. All three factors can be determined, since the joints are detect­ able features whose characteristics can be quantitatively measured and described. Joints are universally present in rock masses and have strength, per­ meability and deformational characteristics appreciably different from those of the intact rock. Depending upon the origin of the joints sets, their characteristics can vary greatly. Not only can the average spacing between joints vary within wide limits, but the nature and degree of joint infilling material, physical characteristics of their planes and their degree of develop­ ment can be vastly different. Because of variations in these properties one joint set can have very different effects than another on, for example, shear characteristics. Hence, each joint set should be examined individually for its properties.