Wavelength division multiplexing or WDM allows the combining of a number of independent information-carrying wavelengths onto the same fiber, because of the wide spectral region in which optical signals can be transmitted efficiently. This chapter addresses the operating principles of WDM, describes a wide array of optical sources and passive components needed to implement WDM links, examines the functions of a generic WDM link, and discusses the internationally standardized spectral grids that designate independent channels for wavelength multiplexing schemes.