Wavelength-division m ultiplexing (WDM) multiplies transmission capacity by allowing a single optical fiber to carry separate signals at multiple wavelengths, but that benefit comes at a cost in complexity. Additional optical components are needed to combine and separate optical channels at closely spaced wavelengths, and to process the signals as they are transmitted. Specific requirements vary with system design, and several technologies are in use. This chapter first outlines the basic requirements, then explains how these requirements relate to the operation of WDM systems. Then it covers specific technologies, their operation, and their capabilities. The chapter introduces the important concepts of wavelength-selective optical filtering, and describes the types of devices that can perform it. You will recognize a little overlap with the couplers and attenuators described in Chapter 14, which can be adapted for wavelength selectivity. The current chapter concentrates on passive technologies like those in Chapter 14, but does mention some active technologies that serve similar purposes in “dynamic” versions of passive components. Chapter 16 will cover switching, modulation, and other “active” technologies.