The erbium-doped fiber amplifier (EDFA) has become a key component in many optical networks because it provides efficient, low-noise amplification of light in the optical fiber low-loss telecommunications window near 1550 nm. As more systems using EDFAs have been built and tested, new issues and effects have emerged as critical to the development and expansion of this technology into the future. Single-channel amplification, as described in Chapter 12, is being supplanted by multiple channel wavelength-division-multiplexed (WDM) systems. EDFAs are being used in analog applications, not solely in digital systems. In most new applications, a high-output power and a low-noise figure (NF) are required. To achieve these goals, the emphasis has been placed on both fiber optimization and clever EDFA designs. High-power pump sources have made this task easier, while advances in fiber-grating technologies have facilitated more complex EDFA designs.