Since the early demonstrations in 1987, optical amplifiers based on rare-earth-doped fibers have been widely recognized as key devices for achieving high-performance, high-capacity and long-distance transmission on optical fiber networks. The erbium-doped fiber amplifier is the best-known and most-frequently used optical amplifier. Its mature technology, combined with an outstanding reliability, make it the ideal element for both terrestrial or submarine, analog or digital optical fiber communication systems in the 1.55-mm window. These devices amplify the lightwave signal in the optical domain, unlike electrical repeaters. As a result, they are transparent to bit rates, and they can simultaneously amplify several optical channels. As telecommunications companies have pushed to increase the capacity of their fiber optic transmission lines, the fiber amplifier has enabled an exploding growth in so-called wavelength division multiplexing systems, which multiplex several wavelengths (optical channels) in the 1.55-mm window into a single fiber. Redesigned erbium-doped fiber amplifiers offer even more advantages, such as gain flattening, which will enable practical terabit-per-second all-optical multiwavelength communications in the near future.