This book describes the phenomena that arise from the interaction between quantum systems with their environment. The emerging irreversible dynamics of local systems explains the classical behaviour of macroscopic objects. The emergence of superselection rules, observed particle aspects of quantum fields, the occurrence of quantum jumps, and the emergence of classical spacetime from quantum gravity are also discussed. This approach, which is based on the assumed universality of quantum mechanics, is compared and contrasted with others, such as consistent histories, open-system dynamics, and explicit collapse mechanisms. In particular, the impact of decoherence on the notorious quantum measurement problem is discussed.