This paper provides prospects and current status of quantum-dot semiconductor optical amplifiers (SOAs) and their comparison with bulk and quantum-well technology, based on our pioneering work covering the proposal of their promising features, subsequent experimental demonstrations, and the design of all-optical quantum-dot switching modules. The proposed promising features are diverse; high-saturation power, high-speed amplification up to 160 Gb/s under gain saturation without pattern effect, high-speed cross-gain modulation up to 160 Gb/s without pattern effect, multiple-wavelength processing over broad gain spectra, and symmetric wavelength conversion by four-wave mixing. The operation theory of quantum-dot SOAs is provided in order to treat various aspects unique to quantum dots such as spatial localization, retarded carrier relaxation, and inhomogeneous and homogeneous broadening. Pattern-effect-free amplification at 10-40Gb/s, wavelength conversion by the cross-gain modulation at 10-40Gb/s, and symmetric wavelength conversion by four-wave mixing are experimentally demonstrated in 1.3-micron InGaAs/GaAs quantum-dot SOAs. All-optical quantum-dot switching modules are proposed, which we expect to work in the next-generation flexible all-optical photonic networks.