An optical amplifier is a device which receives some input optical signal and generates an output optical signal with higher power without the conversion to an electrical signal during the intermediate stages. The amplification process takes place in a so-called gain medium. Most optical amplifiers are laser amplifiers. Their gain media are either insulators doped with some laser-active ions or semiconductors. The amplification process in such amplifiers is based on stimulated emission. Moreover, various types of optical non-linearities can be used for optical amplification in addition to stimulated emission. This book presents leading edge research on optical amplifiers from researchers spanning the globe.