The breakthrough technologies that led to the rapid increase in capacity of optical communication systems are EDFA (Erbium-Doped Fiber Amplifier), which emerged in the late 1980s, and Wavelength Division Multiplexing (WDM) technology, which became widespread in the late 1990s. The first long-distance optical amplifier repeater submarine cable system, TPC-5, used a single-channel signal of 5 Gb/s. However, as mentioned in the previous chapter, the bit rate in long-distance transmission was limited to about 40 Gb/s. Therefore, research and development for long-distance, high-capacity systems shifted to WDM systems, which can expand total capacity by increasing the number of wavelengths. In the one of the earliest WDM-based Trans-Pacific optical submarine cable systems, the China-US cable, WDM technology with a channel bit rate of 2.5 Gb/s was introduced. Compared to TPC-5’s 5 Gb/s bit rate, the WDM system had to reduce the channel bit rate. This was due to the difficulty of transmitting high-speed WDM signals over long distances, caused by the fiber nonlinearity. High-speed optical signals are more severely affected by FWM (Four-Wave Mixing) and XPM (Cross-Phase Modulation) between WDM signals. Moreover, as discussed in Chap. 5 , NRZ-OOK signals are not the optimal signal format for nonlinear optical fiber transmission. This chapter describes the basic configuration of WDM systems and various nonlinear suppression techniques developed to increase the channel bit rate from 10 to 40 Gb/s. The commercial WDM-based long-distance optical submarine cable system technologies that incorporate these techniques are also covered.