The content of this book is the result of the work and the experiences of an interdisciplinary and strictly European group of researchers who have attempted to give birth to a new fibre communication network concept by exploiting the potential benefits of optical coherent transmission. The run towards this ambitious goal started in 1985 on the basis of the ESPRIT program incentives by an "ad hoc" consortium of industrial partners and research institutions. The first three years were dedicated to a feasibility study carried out by a reduced number of "pioneers". In 1989 the team was extended to eleven partners. There was already clear evidence of the increasing interest in Europe in exploring the actual limits of optical fibre coherent systems; the project had innovative and very advanced features that were gradually refined by incorporating the latest technological developments to which it has directly contributed. The main objective of the project targeted the development of the necessary building blocks to show experimentally the performance and the flexibility built into the proposed concept of multi-channel ultra-wideband network. The system concept developed within this project associates optical frequency division multiplexing with a suitable network architecture and management techniques to allow very high flexibility and efficiency in handling simultaneous transmission over the network (on each optical frequency) of multiple virtual channels each operating with a wide range of information rates ranging from a fraction of a Mb/s up to a maximum value in excess of 160 Mb/s.