The paper is a general review of double heterostructure GaAlAs and GaInAsP injection lasers and their application in long-distance optical communication systems. After briefly describing the physics of semiconductor laser operation, the performance of oxide stripe and advanced laser structures at 0.85, 1.3 and 1.55 μm wavelength is discussed. The parameters of greatest importance in optical communication systems such as laser spectrum and modulation response are covered in some detail and estimates of device parameters and manufacturing tolerances are given.The relative merits of the different types of laser for both multimode and single-mode optical fibre systems are then discussed. It is shown that single-mode systems operating close to the dispersion minimum are capable of data rates above 390 Mbit/s over distances of 30 km even when fairly conservative worst case assumptions are made. The key parameters that will allow single-mode systems to operate at data rates of more than 1 Gbit/s over distances of 50 to 100 km are identified.