The accretion process is thought to play a key role in the Universe. This book explains, in a form intelligible to graduate students, its relation to the formation of new stars, to the energy release in compact objects and to the formation of black holes. The monograph describes how accretion processes are related to the presence of jets in stellar objects and active galactic nuclei and to jet formation. The authors treat theoretical work as well as current observational facts. This volume of the highly esteemed Les Houches series is meant as an advanced text that can serve to attract students to exciting new research work in astrophysics.