This book about rock stress is suitable for students in geosciences and rock engineering, who need to broaden their horizons about the Stress Field of the Earths Crust. The book covers the topic so that geosciences students will be able to grasp the Cauchy Stress Principle without fear of matrix transformations in an exercise. Students interested in mathematics, physics and engineering will learn how strain gauges are used to obtain in-situ stress by the overcoring method. Leading edge technology in determining rock stress like quadruple packer and the Kaiser effect are presented together with classical methods like hydraulic fracturing. Borehole techniques (breakouts) and core-based methods (anelastic strain recovery) are illustrated. With respect to stress data, we choose locations of global interest where integrated stress determination strategies have been applied like the scientific ultra-deep drilling project KTB (Germany), the excavation for nuclear waste disposal at Olkiluoto (Finland), or the drilling into a seismic active fault zone at SAFOD (USA). Stress compilations viewed by the World Stress Map project are presented and interpreted in terms of plate tectonics. The purpose of this book is twofold. First, to report on the current field of knowledge in rock stress to students at an introduction level and second, to inform readers about stress measurements in underground excavations and boreholes of public interest and about global rock stress in plate tectonic settings. Apart from the introduction, the textbook consists of three major parts. Part I starts with definitions, rock stress terminology, and ends in simple Earth stress profiles (Chapters 2-5). Part II is about stress measuring techniques from an experimental physics basis (Chapter 6) and a rock mechanics viewpoint (Chapters 7 and 8). Part III is about displaying and interpreting stress data at local and global scale with plate tectonics as a fundament (Chapters 9-11).