The backbone of Geotechnical Engineering does not concern the development of more or less sophisticated tools and theories. It remains in a previous step. When facing a real problem it is necessary to isolate its fundamental aspects and to achieve a correct conceptual representation of its nature. This phase requires abstract thinking, which is certainly assisted by a proper understanding of paradigms and theories of Soil and Rock Mechanics. The process of abstract thinking with the aim of identifying the key issues usually renounces to complexity and secondary details. To be successful, concepts should be simple and rooted on well established mechanical and physical knowledge. Only when the relevant mechanisms or phenomena which define the problem are well understood, additional sophistication may be added for a more accurate analysis or interpretation. This book remains in this first "simple" stage.