The rapid growth in demand for high-capacity telecommunication links, and the speed limitation of single-wavelength links, has resulted in an extraordinary increase in the use of wavelength-division multiplexing (WDM) in advanced lightwave networks. In this work we review the basic concepts of WDM, describe its key features and why network providers are using it, review the enabling technologies, discuss the basic operating characteristics and their associated parameters, and show how WDM is being used worldwide in both terrestrial and undersea installations. Included in this are discussions on management functions that are needed for monitoring the status and health of WDM networks. (C) 1999 Academic Press.