All rock masses contain bedding planes, fissures, fractures, joints and other mechanical defects which are here referred to as "discontinuities". The requirement to provide numerical data on discontinuities for engineering design calculations has created the need to apply the mathematical methods of probability theory, statistics, vector analysis and mechanics to a topic that has previously been handled in a largely qualitative way. This book aims to bridge the gap between the descriptive methods of the geologist and the analytical methods of the rock mechanics engineer as applied to discontinuity characteristics. It does not pretend to be the final word on the topic, but rather seeks to explain in simple terms, basic ideas that can be built upon. Advanced undergraduates studying civil, mining and geological engineering and postgraduates pursuing coursework in rock mechanics, soil mechanics and engineering geology should find this a valuable text. It should also serve as a reference tool for practising engineers and geologists faced with the task of collecting, analyzing and applying discontinuity data for the analysis and design of surface and underground excavations in rock associated with a wide range of mining and civil engineering projects. This book should be of interest to undergraduate students studying civil, mining and geological engineering; Msc and PhD students in rock mechanics, soil mechanics, engineering geology, hydro(geo)logy and related subjects; and practising engineers and geologists.