This volume reports the recent theoretical and experimental results on the macroscopic quantum coherence of macroscopic systems. Particular attention has been devoted to Josephson devices. The correlation with other atomic and molecular systems exhibiting a macroscopic quantum behavior is also discussed. The volume chapters provide both a historical overview and recent theoretical background on the topic, as well as information on new experimental results relative to the quantum computing area. In particular, observations of quantum behavior in molecular and magnetic systems, small Josephson devices, and quantum dots are also reported and are particularly stimulating in view of the realization of several possible q-bits. The present volume, far from being exhaustive, represents an interesting update of the subject and we hope that it will be a useful tool in stimulating new experiments.