This chapter discusses the origin of discontinuities and explain how to identify and measure them in the field. It also discusses the effect of discontinuities on the rock mass properties of sandstone, mudstone and andesite at the project site. Joint spacing is defined as the perpendicular distance between the two neighboring joints. It is measured by a ruler and it can vary from a few centimeters to meters. Persistence is the length of a joint that can be quantified by observing the trace lengths of discontinuities on exposed surfaces. Block size can be classified by the volumetric joint count, which is defined as the number of joints per cubic meter of rock mass. Aperture is the open space between two joints. Open or filled joints with large apertures have low shear strength. In rock mechanics, dip direction/dip format is generally used to describe the orientation of discontinuities.