Recent progress in semiconductor laser amplifiers (SLAs), mainly GaInAsP traveling-wave semiconductor laser amplifiers (TWAs) for use in optical fiber transmission systems, is discussed. The status of antireflection coating on laser-diode facets which are indispensable for TWAs is discussed. Reported data on small-signal gain, signal-gain saturation, and noise are summarized and discussed in relation to active-layer parameters. Common amplification using SLAs for the amplification of various multiplexed signals, including interchannel crosstalk, is also described. A thick, short active-layer structure for a broadband high-output-power low-noise TWA with polarization-insensitive signal gain is proposed. >