The III-V AlGaInAs quaternary is particularly well suited to conventional molecular beam epitaxy, and many devices based on this material system have already proved of interest in high-performance microelectronics. However, although this lattice-matched-to-InP quaternary covers the same wavelength range as the well known GaInAsP system, it was not until recently considered as a serious challenger for optoelectronic applications. This is no longer the case, because of theoretical considerations and because of recent device achievements, especially low-threshold MQW laser diodes and high frequency electro-optic modulators.