The development of InGaAsP lattice-matched to InP as a suitable material for a range of electronic devices is reviewed. Currently accepted values of fundamental material parameters such as lattice constant, energy band-gap and effective mass as a function of composition are presented. The various growth techniques are discussed with particular emphasis given to the liquid phase epitaxy (LPE) method which has emerged as the most popular. Details of the determination of the liquidus and solidus phase diagrams both theoretically and experimentally are given and a comparison of the two is carried out. The problems of doping control and lattice matching are discussed. The other less widely-used growth methods, vapour phase epitaxy (VPE) and molecular beam epitaxy (MBE), are also outlined. The development of optical sources (lasers and LED's) and photodetectors for optical-fibre communication systems is presented with particular attention being paid to the device technology. The latest performance figures in this rapidly moving area are reviewed. Prospects for the use of this material in non-optical applications are discussed in terms of its transport properties and device technology for applications in microwaves and high-speed logic.