There have been rapid advances in multiplexer and demultiplexer development driven by the promises of WDM to increase the information-carrying capacity of optical fibers. The design of WDM components selected for a particular system depends on system parameters such as number of channels, direction of channels, channel loss budget, bandwidth, and source type. Multiplexers and demultiplexers can be categorized according to the optical technique used to make the couplers wavelength selective: filters or dispersive elements. Optical design examples show that filter type couplers are best for a small number of channels (3-4) while dispersive type couplers are better for larger numbers of channels. System design examples are given for three types of systems with up to eight channels. Experimental results on WDM multiplexers, 'demultiplexers, and bidirectional couplers are given. Couplers optimized for operation in a single direction have been demonstrated with channel insertion losses <2 dB and crosstalk <40 dB between channels. Couplers for bidirectional operation are shown to have higher losses. The severe effect on system crosstalk performance of out-of-band source emission is discussed.