The high energy electron?positron linear collider is expected to provide crucial clues to many of the fundamental questions of our time: What is the nature of electroweak symmetry breaking? Does a Standard Model Higgs boson exist, or does nature take the route of supersymmetry, technicolor or extra dimensions, or none of the foregoing? This invaluable book is a collection of articles written by experts on many of the most important topics which the linear collider will focus on. It is aimed primarily at graduate students but will undoubtedly be useful also to any active researcher on the physics of the next generation linear collider.