The definition of a discontinuity in this book is taken to be a surface in a rock mass that has the potential to open during engineering works and has lower strength than the surrounding rock (British Standards Institution, 2003). Examples of discontinuities in rock using this definition are bedding or compositional layering, joints and foliations, together with faults and intrusive contacts which are discussed in separate chapters. However, it is recognized that for engineering purposes, it may be more appropriate to restrict the usage of the term discontinuity to where mechanical fracture has taken place (International Society for Rock Mechanics, 1978; Norbury, 2010; Hencher, 2012), which would only include open fractures such as joints. Whatever the definition one uses, it is the recognition, description and testing of discontinuities that are paramount in any rock engineering investigation.