Undersea Fiber Communication Systems, Third edition is a reference guide covering all aspects of optical communication systems over submarine cables, including design, technology, installation, and maintenance, for both the wet and dry plant. For more than 20 years, the book has served as the bible for the submarine optical communications industry, proving useful for all people interested in submarine cables, from students, to experts, operators, and investors. Since the publication of the second edition of the book in 2015, several technical areas have rapidly evolved, and this new third edition keeps the book up to date in this fast-moving field. The key technical areas impacted over the past decade and addressed in this volume include: Transverse role of open cables, from architectures and design to upgrades and management; Evolution of coherent transponders now fully versatile with hundreds of possible modes of operation; Impacts of space division multiplexing (SDM) with the foreseen evolution towards full multi-core fiber solutions in the next decade; Deep evolution of wet plant equipment (repeaters, branching units, cables) supporting SDM and reconfiguration of networks; Environmental impacts of marine operations from laying to cable recovery, leading also to many new developments of SMART cables; Contract management for laying open cables as well as for recovery of end-of-life cables; Unrepeatered systems that have progressed to match repeatered systems at network levels.