The book presents an introduction to the mechanical genesis of tectonic faults in the brittle crust of the Earth. In the first chapters mechanical concepts of rock (such as brittleness, stresses in a discontinuum, effective stress, buoyancy, poro-thermo-elasticiy, fracture modes and the corresponding failure and slip conditions) are discussed. The book focuses on the critical re-assessment of Coulomb-Mohr's theory of sliding deformation in rocks. The book concludes with a discourse on similarity and self-similarity of fault structures and a critical examination of the feasibility of scaled model experiments. Mathematical formalism is restricted to a minimum and is replaced, wherever possible, by the extremely useful graphic method of Mohr's stress circle which is introduced in a separate chapter. The book includes illustrative geological and geotechnical examples.