The central theme of this book is the problem of how to explain within quantum theory the classical appearance of our macroscopic world. This topic is of fundamental importance not only for the status of quantum theory but also for the philosophical discussion concerning our view of the physical world. The book originated from the regular Workshop on Foundations of Quantum Theory organized for the past several years at the Forschungsstatte der Evangelischen Studiengemeinschaft (FESt, or Protestant Institute for Interdisciplinary Research) by one of the authors (I.O.S.). These meetings assembled people with longstanding activities in the field of decoherence and people who within their fields of research became interested in the decoherence program as a quantitative approach to basic questions in quantum theory.