Configurations, basic characteristics, and the methods for implementation of wavelength-division multiplexers/demultiplexers as the key elements of the end stations in fiber-optics communication systems are considered. The most important studies directed at practical implementation of wavelength-division multiplexers in specific devices based on SiO2 and InP are reviewed in detail. SiO2-based multiplexers exhibit lower losses, whereas devices based on InP make it possible to use the technologies of microelectronics and integrating optics and to fabricate radiation sources and passive components on the same substrate. Schemes for optical multiplexers with adding and dropping of channels and examples of their implementation are given. It is emphasized that high-density wavelength-division multiplexers have small sizes and are fabricated by conventional lithography, which makes them promising in wideband optical communication systems. ©2000 Kluwer Academic/Plenum Publishers.