This book has its origin in an interdisciplinary graduate class that I've taught at Stanford University for a number of years and a corresponding short course given in the petroleum industry. As befitting the subject matter, the students in the courses represent a variety of disciplines – reservoir engineers and geologists, drilling engineers and geophysicists. In this book, as in the courses, I strive to communicate key concepts from diverse disciplines that, when used in a coordinated way, make it possible to develop a comprehensive geomechanical model of a reservoir and the formations above it. I then go on to illustrate how to put such a model to practical use. To accomplish this, the book is divided into three major sections: The first part of the book (Chapters 1–5) addresses basic principles related to the state of stress and pore pressure at depth, the various constitutive laws commonly used to describe rock deformation and rock failure in compression, tension and shear. The second part of the book (Chapters 6–9) addresses the principles of wellbore failure and techniques for measuring stress orientation and magnitude in deep wells of any orientation. The techniques presented in these chapters have proven to be reliable in a diversity of geological environments. The third part of the book considers applications of the principles presented in the first part and techniques presented in the second.