State of the art optical amplifier technologies are reviewed focusing on research at ntt. The following developments will be described : an erbium-doped silica fiber with a very high gain coefficient of 11.0 dB/mW, a very small (volume : 36 cc) fiber amplifier module pumped by a 980 nm InGaAs laser diode, an Er-doped fiber amplifier pumped by an AlGalnP visible laser diode, praseodymium-doped fluoride glass fiber amplifiers operating in 1300 nm band, silica-based erbiumdoped planar lightwave circuit amplifiers, and their applications in various optical transmission and measurement systems. The future trends in the optical amplifier research are also described.