The alloy system A1GaAs/GaAs is potentially of great importance for many high-speed electronics and optoelectronic devices, because the lattice parameter difference GaAs and A1GaAs is very small, which promises an insignificant concentration of undesirable interface states. Thanks to this prominent feature, a number of interesting properties and phenomena, such as high-mobility low-dimensional carrier gases, resonant tunnelling and fractional quantum Hall effect, have been found in the A1GaAs/GaAs heterostructure system. New devices, such as modulation-doped FETs, heterojunction bipolar transistors, resonant tunnelling transistors, quantum-well lasers, and other photonic and quantum-effect devices, have also been developed recently using this material system. These areas are recognized as not being the most interesting and active fields in semiconductor physics and device engineering.