Wavelength Division Multiplexing (WDM) couplers are specifically designed for wavelength division multiplexing applications, where multiple optical signals of different wavelengths are combined onto a single fiber or separated into individual fibers. This is often compared to using a fiber as a single-lane road, where each service requires its own path. They play a crucial role in various applications, such as telecommunications, data centers, and fiber-to-the-home (FTTH) installations. This chapter addresses the operating principles of WDM.