Wavelength division multiplexers are fundamental to the functioning and performance of integrated photonic circuits, with applications ranging from optical interconnects to sensing and quantum technologies. In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i. Each wavelength, or “channel,” carries an independent data stream, allowing bandwidths up to 400. Wavelength division multiplexing (WDM) can help network operators stay ahead of growing demand for bandwidth. This chapter addresses the operating principles of WDM.